How should the following sentence be completed: Last summer, __________ packed a tent, sleeping bags, and a cooler filled with barbeque and headed to the beach with friends.

The correct completion of the sentence is "my cousin and I." This choice is appropriate because when referring to oneself and another person as the subject of the sentence, the pronoun "I" is the correct form to use. Subjects perform the action of the verb, which in this case involves packing items and heading to the beach.

In context, the phrase "Last summer, my cousin and I packed a tent..." indicates that both the speaker and their cousin took part in the action, making "I" the grammatically accurate choice.

Using "I" instead of "me" is essential in this construction because "I" is the subject form of the pronoun, while "me" is the object form. Since the sentence requires the subject form, "I" fits perfectly.

Additionally, the phrase needs to maintain a natural flow, which is accomplished through the correct grammatical structure.