The correct passive form of the given sentence is:

☀ The lock was found broken by us last night.

Let's break down the options:

1. The lock was found by us breaking last night. (Incorrect) This sentence implies that "we" were breaking something last night, not that "we" found the lock broken.

2. The lock was found broken by us last night. (Correct) This sentence correctly uses the passive voice and conveys the meaning that "we" discovered the lock in a broken state last night.

3. The lock was broken by us last night. (Incorrect) This passive sentence implies that "we" were the ones who actively broke the lock, which changes the meaning of the original sentence.

4. The broken lock we found last night. (Incorrect) This is not a passive sentence. It is an active sentence with a relative clause, but it does not express the same idea as the original sentence.