Shahzain had already ____ (swim) five laps when I jumped into the pool.

The correct past participle to fill in the blank is swum:

"Shahzain had already swum five laps when I jumped into the pool."

Here's why:

• "Swim" is the base verb and wouldn't work in this context because it's the present tense.

• "Swum" is the past participle of "swim" and is used with the auxiliary verb "had" to form the past perfect tense, indicating an action completed before another past action (jumping into the pool).

• "Swimmed" is another past tense form of "swim," but it wouldn't fit correctly here because the sentence requires the past perfect tense to show that Shahzain's swimming happened before your jump.

• "Swimming" is the present participle and wouldn't fit the grammatically needed past perfect tense form.

Therefore, "swum" is the most accurate and grammatically correct choice to convey that Shahzain had completed five laps before you joined the pool.