Grammar: Correct Idiomatic Phrasing

The question tests the correct usage of the English idiom "can't help + verb-ing", meaning unable to stop oneself from doing something.

Idiom Explanation

• The standard idiomatic structure is: $couldn't help$ + gerund (the $-ing$ form of a verb).
• In mathematical notation, the structure is: $\text{Subject} + \text{couldn't help} + \text{verb-ing}$.

Analysis of Options

• Option 1: $could help laughing$ - Incorrect. This suggests the ability to help laughing, not the inability. The idiom requires negation.
• Option 2: $couldn't help laughed$ - Incorrect. The structure requires a gerund ($laughing$), not the past participle ($laughed$).
• Option 3: $couldn't help laughing$ - Correct. This perfectly matches the idiom structure: $couldn't help$ + $laughing$ (gerund).
• Option 4: $could helped laughed$ - Incorrect. This is grammatically incorrect. $Could$ should be followed by a base verb, not $helped$, and the following verb should be a gerund, not $laughed$.

Conclusion

Based on the correct idiomatic structure, the sentence should be: "The movie was funny and I couldn't help laughing."