Correct Answer: useless

Solution : The most appropriate choice is the first option.

The word useless means serving no purpose or not producing any meaningful results. In the context of the sentence, vain means that the efforts will not achieve the desired outcome if the project report is not submitted on time. So, the word useless is the most appropriate synonym to convey this meaning.

The meanings of the other options are as follows:

• Careless: Lacking proper attention or consideration.

• Delightful: Causing pleasure or joy, often related to something being charming or enjoyable.

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• Sensible: Showing good judgement, practicality, or reason.

Therefore, the complete sentence is: The efforts will be useless if we do not submit the project report on time.