On (A={1,2,3,4,5,6}), (aRb) if and only if (a) and (b) are both multiples of (3) or both not multiples of (3). What is the equivalence class of (3)?

Correct answer: ({3,6})

Step 1: (3) itself is a multiple of (3). Step 2: Its equivalence class contains exactly those elements that are multiples of (3). Step 3: In the given set, the multiples of (3) are (3) and (6), so the class is ({3,6}).