I have seen this online.
"The Queen had a birthday, but held no public celebration due to the coronavirus lockdown. What birthday was it? a. 92nd b. 94th c. 96th"
The sentence "What birthday was it?" sounds different to me. I have never seen the word "what" being used for asking about ordinal number of something. "What birthday" sounds as if you are asking about the type of birthday like "an open air birthday party" or "a birthday at a special place", etc.
As a non-native speaker I would probably say "Which birthday was it?" or "What number birthday was it? if I am expecting a ordinal number as an answer.
So, I want to make sure if it is perfectly ok to use "what" like this to ask about the "ordinal number of something. And I want to ask;
Is "What birthday was it?" more idiomatic than "Which birthday was it?" or "What number birthday was it"?