It was a funny episode, I ----------------

Analyze the question: The sentence is "It was a funny episode, I __." We need the correct form of the idiom can't/couldn't help + verb.

Concept: The expression "can't help" or "couldn't help" is followed by a gerund (verb + -ing), not an infinitive.

• I can't help laughing. ✔
• I can't help to laugh. ✘
• I can't help laugh. ✘

Checking each option:

• A. Can't help to laugh: Wrong, because "to laugh" is an infinitive. The tense also doesn't match "was".
• B. Couldn't help laugh: Wrong, because the bare verb "laugh" is not used after this phrase.
• C. Couldn't help laughing: Correct. The past tense "couldn't" agrees with "was", and the gerund "laughing" follows the idiom.
• D. Couldn't help to laugh: Wrong, because it uses an infinitive.

Answer: The complete sentence is "It was a funny episode, I couldn't help laughing.

Exam tip: Remember that can't help is always followed by -ing. Similar patterns include can't stand, don't mind, feel like, and look forward to, which are all followed by the -ing form. Also match the tense to the sentence: "was" goes with "couldn't".