The (-3) charge on Y suggests that an atom of Y requires 3 electrons to fill its outer shell.
The ion Y³⁻ has an electronic configuration of 2, 8, 8.
Since Y has gained 3 electrons to form Y³⁻, Y must have an electronic configuration of 2, 8, 5 (8 – 3 = 5)
The number of outermost electrons (in this case there are 5) determines which group the element belongs to. Y belongs to group 5.
The number of shells (in this case there are 3:1 that holds 2 electrons, second that holds 8 electrons and the third which holds 5 electrons) determines which period the element is in. Hence, Y is in period 3.
Hence the answer is C.