To answer this question, you need to understand the correct use of conjunctions.
Option A) Whether - This option is incorrect because "whether" is used to introduce two or more alternatives or possibilities, not to introduce a condition or reason.
Option B) Though - This option is incorrect because "though" is used to show contrast or concession, not to introduce a condition or reason.
Option C) If - This option is correct because "if" is used to introduce a condition or reason. In this sentence, "if" is used to express a hypothetical condition ("If I have enough time") that is required for the action of watching TV every night.
Option D) Sentence is complete without the blank - This option is incorrect because the sentence requires a conjunction to properly connect the two clauses ("I have enough time" and "I watch TV every night"). Without a conjunction, the sentence would be a run-on sentence.
The correct answer is C) If. This option is correct because "if" is used to introduce a condition or reason.