If A = {1, 2, 3} and B = {4, 5, 6}, which relation is a function from A to B?

Correct answer: {(1, 4), (2, 5), (3, 6)}

To be a function from A to B, every element of the domain A must appear exactly once as a first component, and its second component must be an element of B. Option A has first components 1, 2, and 3, each appearing once, with images 4, 5, and 6; all three images belong to B. Thus it satisfies the function definition. In option B, input 1 is paired with both 4 and 5, while input 3 is missing, so it fails. Option C omits domain element 3, and option D omits domain element 1. A function need not use every element of B, but it must assign one image to every element of A. Therefore A is the only correct answer.