She stayed a __________ throughout her entire life.

Explanation

The term 'spinster' (Option A) specifically refers to a woman who has never married, which fits the context better than the other options. 'Bachelor' (Option B) typically describes an unmarried man. 'Unwed' (Option C) and 'single' or 'alone' (Options D) are more general terms but do not carry the same precise meaning as 'spinster' in this context.