2) B

This question deals with valencies.

X, being in group 3, has a valency of +3. It has 3 electrons in its outer shell. It needs to donate its 3 electrons to be stable.

Y is in group 6, meaning it needs 2 electrons to be stable. Its valency is -2.

To find the formula we can use the following technique:

X                                 Y

                X

+3                               -2

The big X is a criss cross, meaning -2 goes to X and +3 goes to Y.

So the formula is X₂Y₃.