Let' s start with "Spinster" "Spinster" refers to a female person, who has already reached the age of marriage and yet not married. I would not use this word because it is pretty insulting. "Old maid" has the same meaning too. "Bachelor" refers to a male person, and is the male version of "Spinster". I think it is quite a neutral term, and is not as insulting as "Spinter". As for "single". "Single" is someone, male or female, who is just simply not married.

I think also, to add - I would consider bachelor as almost having a positive connotation; meaning "carefree and single". Bachelors are usually considered free, easy-living, having fun. Think of the collocation "eligible bachelor". Interesting that it could be considered a direct opposite to spinster (in my opinion).

I found it in a online visa application form. I'm gonna choose "Others" and specify it with "Single" instead.

Sern is right that you cannot talk about "a single", but strangely enough we can talk about "singles", e.g. "this is a bar where singles go to have fun."

Spinster (for female) Bachelor (for male) Single (for female or male)