Select the most appropriate option to fill in the blank and make a grammatically correct sentence.
The more tired you are, the ________ it is to concentrate.

Options

A. i) harder
B. ii) hardest
C. iii) more harder
D. iv) hard

Explanation

harder should be used.
RULE: Comparison and contrast are expressed by the use of the ….... the …..... with comparative adjectives in parallel clauses.
In this sentence, the parallel clauses are “the more tired you are and “the harder is to concentrate”.