Symmetric relation

Step 1: In option A, every non-diagonal pair has its reverse. Step 2: It does not contain all diagonal pairs, yet symmetry holds. Step 3: Keep the conditions of symmetry and reflexivity separate.

Step 1: Transitivity gives ((a,c)) from ((a,b)) and ((b,c)). Step 2: Here ((1,2)) and ((2,1)) give ((1,1)). Step 3: In mixed-property questions, think by chaining ordered pairs.

Step 1: When (a\ne b), the reverse of ((a,b)) is a different pair ((b,a)). Step 2: This reverse pair is required for symmetry. Step 3: For non-diagonal pairs, do not forget to write the reverse separately.

Step 1: (a<b) is a one-direction rule. Step 2: If (a<b), then generally (b<a) is not true. Step 3: Rules depending on order often break symmetry.

Step 1: Having every ((a,a)) is the condition of reflexivity, not symmetry. Step 2: Symmetry asks only for reverses of existing pairs. Step 3: Distinguish between always true and not necessarily true statements.

Step 1: ((1,2)) and ((2,1)), as well as ((1,3)) and ((3,1)), are reverse pairs. Step 2: ((1,1),(2,2),(3,3)) are missing, so it is not reflexive. Step 3: Check symmetry first and reflexivity separately.

Step 1: All diagonal pairs ((1,1),(2,2),(3,3)) of (A) are present, so the relation is reflexive. Step 2: Both ((1,2)) and ((2,1)) are present, so it is also symmetric. Step 3: When two properties are asked, verify both fully.

Step 1: In a symmetric matrix, (m_{12}=m_{21}) must hold. Step 2: In option A, both entries are (1), so the matrix is identical about the main diagonal. Step 3: In small matrices, compare entries above and below the diagonal.

Step 1: The reverse of ((3,3)) is ((3,3)) itself. Step 2: Therefore no new reverse pair is needed because of this pair. Step 3: Treat diagonal pairs as immediately safe for symmetry.

Step 1: If (a\ne b), then (b\ne a) is also true. Step 2: So with ((a,b)), the reverse ((b,a)) also belongs to the relation. Step 3: This inequality rule remains the same in both directions.

Step 1: In addition, (a+b=b+a). Step 2: Therefore, if (a+b\le 5), then (b+a\le 5) also holds. Step 3: In sum-based inequalities, changing order does not change the condition.

Step 1: (1+2=3), which is odd, so ((1,2)) belongs to the relation. Step 2: (2+1=3) is also odd, so ((2,1)) also belongs. Step 3: In sum-based rules, the reverse pair usually satisfies the same condition.

Step 1: Every given non-diagonal pair has its reverse present. Step 2: Diagonal pairs may or may not be present; they are not compulsory for symmetry. Step 3: In symmetry, check reverses only for existing pairs.

Step 1: The value of (|a-b|) does not change after swapping the order. Step 2: Therefore, if (|a-b|=4), then (|b-a|=4) too. Step 3: Treat absolute difference as a quick clue for symmetry.

Step 1: One counterexample is enough to disprove symmetry. Step 2: You need a pair whose reverse is not in the relation. Step 3: In a long list, first search for one missing reverse pair.

Step 1: All diagonal pairs are present, so they are not the problem. Step 2: The reverse of ((2,3)), which is ((3,2)), is not in the relation. Step 3: Even when reflexivity appears to hold, check symmetry separately.

Step 1: (a+b) and (b+a) are always equal. Step 2: If a pair gives sum (6), its reverse will also give sum (6). Step 3: Connect the commutative property of addition with symmetry.

Step 1: ((3,3)) is its own reverse, so it is fine. Step 2: The reverse of ((1,2)), which is ((2,1)), is missing. Step 3: For minimum correction, add only the required reverse pair.

Step 1: The pair ((1,2)) and ((2,1)) is complete. Step 2: The pair ((2,4)) and ((4,2)) is also complete, but the reverse ((4,1)) of ((1,4)) is missing. Step 3: In a symmetric extension, every missing reverse pair is added.

Step 1: The basic condition of symmetry is based on reverse pairs. Step 2: Therefore the main check is whether the reverse of every present pair belongs to the relation. Step 3: Do not decide only by counting pairs or looking at the size.