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.

harder

The most appropriate option to fill in the blank is harder.

The sentence is in the comparative form, which means that it is comparing two things. In this case, it is comparing the difficulty of concentrating when you are tired to the difficulty of concentrating when you are not tired. The comparative form of the adjective "hard" is "harder."

The other options are not grammatically correct.

• Hardest is the superlative form of the adjective "hard." It is used to compare three or more things. In this case, there are only two things being compared, so "hardest" is not the correct form to use.
• More harder is not a grammatically correct word. The correct form is "more hard."
• Hard is the positive form of the adjective "hard." It is not the correct form to use in this sentence, because it is not comparing anything.