Explanation:

- The sentence states that the efforts will be "vain" if the project report is not submitted on time. This implies that the efforts will be wasted or ineffective without timely submission.

- Option 1: useless - This is a suitable synonym for "vain," as both words imply something that lacks effectiveness or fails to achieve the desired result.

- Option 2: careless - This word refers to a lack of attention or caution. It does not convey the same meaning as "vain."

- Option 3: delightful - This term describes something that is pleasing or brings joy, which is unrelated to the notion of being "vain."

- Option 4: sensible - This expression means showing good judgment or being practical. It does not align with the meaning of "vain."