The correct passive form of the given sentence is:

☀ Much time is wasted by students watching useless videos.

 Let's break down the options:

1. Much time was wasted by students by watching useless videos. (Incorrect) This option uses the past tense "was wasted," which changes the meaning of the original sentence. The original sentence describes a habitual or ongoing action, so it is appropriate to use the present tense "is wasted."

2. Much time will be wasted by students watching useless videos. (Incorrect) This option uses the future tense "will be wasted," which changes the meaning of the original sentence. The original sentence describes a current situation, not a future event.

3. Much time is wasted by students by watching useless videos. (Correct) This sentence correctly uses the passive voice and conveys the same meaning as the original sentence. It uses the present tense "is wasted" to describe the ongoing and habitual action.

4. Much time is wasted by students watching useless videos. (Incorrect) This option is very close to the correct form but omits the second "by" before "watching useless videos." The correct passive form should include both instances of "by" to indicate the agent causing the action.