Correct option is A
The most appropriate synonym of the bracketed word "vain" in the sentence is (a) "useless."
"Useless" conveys a similar meaning, indicating that the efforts will not yield the desired outcome.
Example: All our efforts will be in vain if we arrive too late to qualify.
Synonym: Useless
Useless: Not fulfilling or not expected to achieve the intended purpose or desired outcome.
Antonyms: useful, effective, fruitful, worthwhile.
Meanings of all other options:
Careless: Lacking care or attention; not taking proper precautions.
Delightful: Highly pleasing or enjoyable; bringing delight or joy.
Sensible: Showing good judgment and practicality; reasonable and logical.