'what' vs. 'which'

[mastrochili]

English Grammar Tests, Elementary Level

ESL/EFL Test #93 "Why Questions", question 1

......... countries have a coastline on the Mediterranean Sea?

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

English Grammar Tests, Elementary Level

ESL/EFL Test #93 "Why Questions", answer 1

Which countries have a coastline on the Mediterranean Sea?

Correct answer: (b) Which

Your answer was: incorrect
What countries have a coastline on the Mediterranean Sea?
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what is the difference between "what" and "which"

[Mister Micawber]

.
Which is preferred when there are a limited number of available options.
.
_________________
Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mr. Micawber