The question would be more clear if instead of Equivalence classes, it would have written as no of Partitions.
Equivalence classes are wrt individual element and not a whole relation.
[x] = Equivalence class of element x in relation R.
Now, let come to the given question.
[1] = {1,5}
[2] = {2,3,6}
[3] = {2,3,6}
[4] = {4}
[5] = {1,5}
[6] = {2,3,6}
Total no of partitions are 3.
(1,5) (2,3,6) (4) ANSWER