According to the wikipedia page for Singapore , the proper demonym in this case is "Singaporean".
Admittedly, you see plenty of patterns emerge for demonyms but there is no guaranteed rule, which means you simply must check if you're unsure.
Just know that the proper term for what you're looking for is "demonym."
ell.stackexchange.com/users/65434/Neil
Singaporean is a nationality.
Singapore is a country.
It wouldn't be grammatically incorrect to answer "Singapore", but it wouldn't fit the question as well as "Singaporean".
ell.stackexchange.com/users/37000/Especially Lime
The correct response in the example case would be both technically, "Singaporean" and "Singapore" are both correct but preferably and more understandably, "Singaporean" would be the most common one for use.
ell.stackexchange.com/users/83490/Veraen
It's both.
Both "Singapore" and "Singaporean" are incomplete sentences, that is, they lack a verb.
(Incomplete sentences are widely used in English; using them is not an error.)
To make them complete, we would add a subject and a verb to know for sure what's coming on there: I am from Singapore.
I am Singaporean.
Both are perfectly fine responses, and in this case, you are just making them incomplete by throwing away unnecessary information.
ell.stackexchange.com/users/70693/r5ha
A single word can't be grammatically incorrect.
The correct answer to the question though, would be any of the following:
My nationality is Singaporean.
I am from Singapore.
I am Singaporean.
ell.stackexchange.com/users/83498/Based
Which of these is grammatically correct in response to  "What is your nationality?" ;  “Singapore”  or  “Singaporean” ?
What is the word for your nationality if you are from Singapore?
ell.stackexchange.com
<grammar><word-usage><adjectives><nouns>
ell.stackexchange.com/questions/182332
ell.stackexchange.com/users/41286/Hosain Sadeqi
Singaporean is a nationality. Singapore is a country. One may say "My nationality is Singaporean" or "I am from Singapore". Singaporean is the nationality, Singapore is the country. You could answer 'Singapore' to this question, but it would be a short form of "I am from Singapore" rather than "I am Singaporean".
Singaporean is the nationality, Singapore is the country. You could answer 'Singapore' to this question, but it would be a short form of "I am from Singapore" rather than "I am Singaporean".