Explanation:
The correct answer is option B) "had already left."

Context:
The sentence is describing an action that happened in the past. The speaker is talking about their arrival at the gym and what they found when they got there.

Past Perfect Tense:
The past perfect tense is used to describe an action that happened before another action in the past. In this case, the action of leaving the gym happened before the speaker arrived at the gym.

Past Perfect Tense Structure:
The past perfect tense is formed using the auxiliary verb "had" followed by the past participle of the main verb.

Analysis of Options:
a) "left" - This is the simple past tense, which does not indicate that the action happened before the speaker arrived. Incorrect.
b) "had already left" - This is the correct answer because it uses the past perfect tense to indicate that the action of leaving the gym had already happened before the speaker arrived. The use of "already" emphasizes that the action happened before a specific point in the past.
c) "had been leaving" - This is not grammatically correct. The past perfect continuous tense is not suitable in this context. Incorrect.
d) "was left" - This is the simple past tense, which does not indicate that the action happened before the speaker arrived. Incorrect.

Conclusion:
Based on the analysis, the correct answer is option B) "had already left." This choice correctly uses the past perfect tense to indicate that the action of leaving the gym had already happened before the speaker arrived.