Why we have to use "which" in this sentence?

English Grammar Tests, Elementary Level

ESL/EFL Test #93 [color=blue]“Why Questions”, question 8

Caribbean island does Sir Vivian Richards come from?

(a) Which
(b) What
(c) Where
(d) Who

English Grammar Tests, Elementary Level

ESL/EFL Test #93 [color=blue]“Why Questions”, answer 8

Which Caribbean island does Sir Vivian Richards come from?

Correct answer: (a) Which

Your answer was: [color=red]incorrect
Where Caribbean island does Sir Vivian Richards come from?
[size=200]_________________________[/size]

I dont know why we have to use “which” in this sentence?

Alex Anna

Hi,

Interrogative ‘which’ is used here to indicate that the question is asked about the particular island he comes from.

A