The correct passive form of the given sentence is:

☀ Let a surprise party be organized by us.

Let's break down the options:

1. Let a surprise party be organized by us. (Correct) This option correctly uses the passive voice with the "Let" construction, indicating a polite request for a surprise party to be organized by "us."

2. Let a surprise party be organized. (Incorrect) This option is almost correct, but it omits the agent "by us." In the passive voice, the agent is typically included to indicate who will perform the action. The correct passive form should include "by us."

3. Let a surprise party organized by us. (Incorrect) This option is not in the correct passive voice structure. It is missing the "be" and the past participle of the verb "organize."

4. A surprise party is to be organized. (Incorrect) This option is in the passive voice, but it does not include the "Let" construction as present in the original sentence. The correct passive form should use the "Let" construction as shown in option 1.