The correct passive form of the given sentence is:

☀ Unsolicited advice should not be given.

Let's break down the options:

1. Unsolicited advice is not to be given. (Incorrect) This option uses the present tense "is not to be given," which changes the meaning slightly. The original sentence uses the modal verb "should," which implies advice on what is right or best to do, while this option implies advice on what is allowed or permitted. The correct form should retain the "should" as shown in option 4.

2. Unsolicited advice can't be given. (Incorrect) This option uses "can't" (cannot), which changes the meaning of the original sentence. The original sentence suggests that one should refrain from giving unsolicited advice, whereas this option suggests that it is not possible to give unsolicited advice.

3. Unsolicited advice may not be given. (Incorrect) This option uses "may not be given," which implies that one is not allowed or permitted to give unsolicited advice. The original sentence uses "should," which suggests advice on what is right or best to do, not advice on what is allowed or permitted.

4. Unsolicited advice should not be given. (Correct) This sentence correctly uses the passive voice and conveys the same meaning as the original sentence. It emphasizes that one should refrain from giving unsolicited advice, which aligns with the intent of the original sentence.