Find out whether there is any grammatical error in below sentence.
I have had to work / at the fountain for almost / ten hours before it could / start functioning well.

The sentence begins with present perfect tense in the phrase “I have had to work”, which indicates an action extending up to the present.

However, the clause “before it could start functioning well” uses past tense, which creates a tense inconsistency in the sentence.

For grammatical correctness, the verb should agree with the present perfect construction, and it should be “before it can start functioning well” instead of “could”.

Hence, the grammatical error lies in the part “ten hours before it could”, making Option C the correct answer.