Topic: Right Form of Verb,

Best Answer Explained in Below-:

In the sentence "He couldn’t help (write) the letter," the correct form to complete the phrase is "writing." This stems from the structure of the verb phrase "couldn't help." In English, the expression "couldn't help" is followed by a gerund (the -ing form of a verb) rather than an infinitive or any other verbal form.

The phrase can be understood as meaning that he was unable to stop himself from performing the action. Therefore, when we say "He couldn’t help writing the letter," it conveys that he felt compelled to write it, highlighting a lack of control over the desire to do so.

By contrast, the other options do not fit grammatically:
- A) "to write" (infinitive form) is incorrect following "couldn't help."
- B) "wrote" (past tense) does not fit the intended meaning in this construction.
- D) "written" (past participle) is also not suitable in this context.

Thus, "writing" is the only option that correctly reflects the grammatical and contextual requirements of the sentence.