The correct expression for this sentence is "couldn't help laughing". The phrase "couldn't help" is always followed by the verb-ing form of another verb (gerund).
- Option (A): "could help laughing" is incorrect because "could help" is not typically used in this context.
- Option (B): "couldn't help laughed" is incorrect because "laugh" should be in the gerund form (laughing).
- Option (D): "could helped laughed" is incorrect because "helped" is the past tense, and we need the base form "help" followed by the gerund form.
Therefore, the correct phrase is "couldn't help laughing", making (C) the correct answer.