On (A={1,2,3,4}), (aRb) if and only if (a^2-b^2) is divisible by (3). What is the equivalence class of (1)?

Correct answer: ({1,2,4})

Step 1: Look at the remainders of squares on division by (3). The squares (1^2,2^2,4^2) leave remainder (1). Step 2: (3^2) leaves remainder (0), so (3) is not in this class. Step 3: Numbers with the same square remainder belong to the same equivalence class.