Topic: Conditional Sentence,

Best Answer Explained in Below-:

In the sentence 'If I had a pen, I (write) a letter,' we are dealing with a conditional statement, specifically the second conditional. This type of conditional is used to discuss hypothetical or unreal situations that are unlikely to occur in the present or future.

The structure of the second conditional typically involves using the simple past tense in the 'if' clause (in this case, "If I had a pen") and the modal verb "would" followed by the base form of the verb in the main clause. This is exemplified by option B: "I would write a letter."

Now, let's analyze the other options briefly:

- Option A: "writing" is a gerund and does not fit the grammatical structure needed here.
- Option C: "will wrote" is incorrect as "will" should be followed by the base form of the verb, and "wrote" is the past tense of "write."
- Option D: "had written" is in the past perfect tense, which doesn't apply to this hypothetical scenario.

Thus, option B "would write" is the only choice that correctly aligns with the grammatical rules of the second conditional, making it the right answer.