Element X has three electrons in its outermost orbit.
Element Y has seven electrons in its outermost orbit.

What is the chemical formula of the ionic compound
formed by these elements when they react together?
(Use X and Y as the chemical symbols.)

A has 3 electrons in outermost orbit and B has 6 electrons in its outermost orbits. So A can give three electrons to complete its octet and B need 2 electrons to complete its octet. so 2 atoms of A will release 6 electrons and 3 atoms of B will need six electrons to complete their octet So, the formula will be A2 B3.

i'm assuming that the correct answer is indeed the first one/ "a"..

b. accuracy of the data matters alot more than its number of digits, organization or amount of detail. quantitative observation is more accepted than the qualitative observation.