Both sentences are grammatically correct, but the choice between "marks" and "mark" depends on the context and whether you are referring to a single numerical score or a set of scores. "She got top marks in math": This suggests that she performed exceptionally well and received high scores on multiple aspects or questions within the math test. "She got top mark in math": This implies that she received the highest possible score overall on the math test. If there is only one test, and you're referring to the overall score, then "She got top mark in math" would be more appropriate. However, in some English-speaking regions, it's common to use "marks" even when talking about a single score. Both could be considered acceptable, but the nuance might vary depending on regional or personal preferences.