Select the most appropriate option to fill in the blank.

As there were reports of manipulation of scores in the previous exams, the aspirants demanded a process to ensure _____ assessment for selection.

fair and square

The most appropriate option to fill in the blank is fair and square.

The sentence is about the aspirants' demand for a fair and transparent assessment process, in light of the reports of manipulation of scores in the previous exams. The phrase "fair and square" means honest and impartial. It is the most appropriate word to use in this context, as it describes the type of assessment process that the aspirants are demanding.

The other options are not as appropriate:

• Will o' the wisp means something that is elusive or illusive. It would not be accurate to use "will o' the wisp" in this context, as the aspirants are not demanding an elusive or illusive assessment process.
• Hole and corner means secret or underhanded. It would not be accurate to use "hole and corner" in this context, as the aspirants are not demanding a secret or underhanded assessment process.
• Well off means wealthy or prosperous. It would not be accurate to use "well off" in this context, as the aspirants are not demanding an assessment process that will make them wealthy or prosperous.