In the following question, some part of the sentence may have errors. Find out which part of the sentence has an error and select the appropriate option. If a sentence is free from error, select “No error”. Did your (1)/ best to keep (2)/ your grades up.(3)/ No error (4).?

Option 1 : 1

The correct answer is '1' i.e. error lies in the first part of the sentence.

In part 1 of the sentence, the usage of 'did' is incorrect Instead, use 'do'

• The imperative is formed using the bare infinitive form of the verb. 
• Imperatives are used principally for ordering, requesting, or advising the listener to do (or not to do) something: "Put down the gun!"; "Pass me the sauce"; "Don't go too near the tiger." They are also often used for giving instructions as to how to perform a task ("Install the file, then restart your computer"). They can sometimes be seen on signs giving orders or warnings ("Stop"; "Give way"; "Do not enter"). 
• The imperative sentences are always written in the present tense hence, this sentence should also be written in the present tense.

So the correct sentence is- Do your best to keep your grades up.