Concept:The phrase "It's about time" is used to say that something is overdue and should happen soon.
It is followed by a past tense verb, even though the meaning refers to the present or future.
Explanation:After the pattern "It's about time" or "It is time", English requires the past simple form.
This is known as the past subjunctive, used for imaginary or desirable situations.
The base verb here is "start".
Its past simple form is "started".
So the correct sentence becomes: "It's about time we started studying hard."
Options A, B, and D do not follow this rule and are grammatically incorrect in this structure.
Example: "It's about time our country invested in education."
Here, "invested" is the past tense used after "It's about time".
Thus, the verb "start" must also change to "started".
Answer:C. started