Answer:

When completing the sentence "I will call you when I ...... back," we need to use the correct future form of the verb. Let's analyze the options given:

a) comes: This is the simple present tense form of the verb "come," which is not appropriate for this sentence. It does not indicate a future action.

b) come: This is the base form of the verb "come," also known as the infinitive. While it is grammatically correct, it does not convey the future action that is required in this sentence.

c) will come: This is the correct option. "Will come" is the future simple tense of the verb "come." It indicates that the action of coming will happen in the future. This form is commonly used to express intentions, predictions, or promises.

d) had come: This is the past perfect tense of the verb "come," which is not suitable for this sentence. The context of the sentence does not require a past action.

Therefore, the correct answer is option "c) will come."

Key points:
- The sentence requires a future action, so the future simple tense is needed.
- The correct future form of the verb "come" is "will come."
- The options "comes" (simple present tense) and "had come" (past perfect tense) are not appropriate for this sentence.
- The base form "come" (infinitive) is grammatically correct but does not convey the future action.

Conclusion:
To correctly complete the sentence "I will call you when I ...... back," the appropriate future form of the verb "come" is "will come" (option c). This form indicates that the action of coming will happen in the future.