The tense used to show the completion of an action by a certain time in the future is the Future Perfect Continuous Tense.

Explanation:

The Future Perfect Continuous Tense is used to describe an action that will have been ongoing up until a certain point in the future. It is formed by using the auxiliary verb "will" + "have been" + the present participle (-ing form) of the main verb.

Example: By 8 pm tonight, I will have been studying for 5 hours.

In this example, the action of studying will continue until 8 pm tonight, and at that point, it will have been ongoing for 5 hours. Therefore, the Future Perfect Continuous Tense is used to show the completion of the action by a certain time in the future.

Option (A) - Future Perfect Tense: This tense is also used to describe an action that will be completed at a certain point in the future but does not necessarily imply that the action is ongoing until that point.

Example: By 8 pm tonight, I will have studied for 5 hours.

In this example, the action of studying may or may not continue until 8 pm tonight. Therefore, the Future Perfect Tense is not the correct tense to show the completion of an action that is ongoing until a certain time in the future.

Option (B) - Present Indefinite Tense: This tense is used to describe an action that is happening right now or regularly.

Example: I study every day.

This tense is not used to describe the completion of an action by a certain time in the future.

Option (D) - Both (A) and (C): This option is incorrect because the Future Perfect Tense is not the correct tense to show the completion of an action that is ongoing until a certain time in the future. Therefore, only option (C) - Future Perfect Continuous Tense is the correct answer.

Present continuous tense