Explanation:

The original sentence is in the active voice, using the modal auxiliary 'had to'. When converting an active sentence with 'had to' to passive voice, the structure becomes 'had to be + past participle'. The object of the active sentence ('all other work') becomes the subject of the passive sentence. Thus, "All other work had to be stopped by us to complete our assignment" is the correct passive form. Option (1) changes the tense and voice incorrectly. Option (2) uses 'had stopped', which is past perfect, not the correct passive structure for 'had to'. Option (4) uses 'was stopped', which is simple past passive, also incorrect for 'had to'.