Answer:

To correctly complete the sentence, we need to choose the verb tense that indicates an action that started in the past and is still ongoing. The correct answer is option B: "will have been watching."

Explanation:

The sentence states that Sam has been watching TV for quite a long time. This implies that the action of watching TV started in the past and is still continuing in the present. To convey this meaning, we need to use the future perfect continuous tense.

The future perfect continuous tense is formed using the modal verb "will" followed by the auxiliary verb "have" and the past participle of the main verb, combined with the present participle "-ing" form of the verb "watch."

Construction of future perfect continuous tense:

Subject + will + have + been + verb-ing

Example:

- I will have been studying for five hours by the time the exam starts.

In the given sentence, "Sam will have been watching TV for quite a long time" indicates that the action of watching TV started in the past and will continue until the present or a specified point in the future. This construction accurately conveys the meaning that Sam has been continuously watching TV for a prolonged duration.

Other options:

- Option A: "been standing" is incorrect because it does not fit the context of the sentence. The sentence is about watching TV, not standing.

- Option C: "none of the above" is incorrect because option B, "will have been watching," is the correct answer as explained above.