If A = {3, 4, 5, 6} and B = {1, 2, 5, 6}, then A ∪ B =

A = {3, 4, 5, 6} & B = {1, 2, 5, 6}

Here we are asked to find A ∪ B

Union means cub the elements of the both the given two sets.

So,

A ∪ B = {3, 4, 5, 6} ∪ {1, 2, 5, 6}

= {1, 2, 3, 4, 5, 6}

∴ option A is the correct answer.