The correct answer is B. Told.

1. Told: This option is correct because it uses the past tense form of the verb "tell," which is appropriate for the second conditional construction. The sentence is expressing a hypothetical situation, and the correct form for the if-clause in the second conditional is the past simple tense.

2. Tell: This option is incorrect because it is in the base form of the verb, which does not fit the conditional construction needed for expressing hypothetical situations.

3. Am telling: This option is incorrect because it is in the present continuous tense, which does not fit the conditional context of the sentence.

4. Will tell: This option is incorrect because it is in the future tense, which is not appropriate for the second conditional construction, where the past simple tense is used in the if-clause.

Summary:

- B. Told is the correct choice because it properly uses the past simple tense to fit the second conditional structure, indicating a hypothetical situation.