Explanation:

- Option 1: "picnic"

This option is incomplete. The sentence lacks an article before the noun, making it grammatically incorrect.

- Option 2: "a picnic"

This is grammatically correct. "A" is an indefinite article used before a singular, general noun.

- Option 3: "picnics"

This option is incorrect due to verb and noun agreement. "Picnics" is plural, and there’s no indication that the class is organizing multiple events.

- Option 4: "the picnic"

This implies a specific picnic known to the listener or reader but isn't ideal if no prior context is provided.

Correct Answer: “Option 2 - a picnic”