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The words are in positive, comparative and superlative form respectively.

Positive- the adjective when used to describe only one thing/person
Comparative- The name itself, form of adjective used for comparing two things/people
Superlative- Used for comparing more than two people.

In this case failing in high school is compared to many other nightmare in his life so we should use the superlative form. 

Therefore the answer is C. and the sentence will be:
Failing in high school could be the worst nightmare in his life