The correct passive form of the given sentence is The man was robbed of all his possessions by someone.

The sentence "Someone robbed the man of all his possessions" is in the active voice. The active voice is formed by putting the subject of the sentence before the verb and the past participle of the main verb. In this case, the subject of the sentence is "someone" and the main verb is "rob." The past participle of "rob" is "robbed." So, the active form of the sentence is "Someone robbed the man of all his possessions."

The passive voice is formed by putting the subject of the active sentence after the verb "to be" and the past participle of the main verb. In this case, the subject of the active sentence is "the man" and the main verb is "rob." The past participle of "rob" is "robbed." So, the passive form of the sentence is The man was robbed of all his possessions by someone.

The other options are incorrect because:

• The man is robbed all his possessions by someone is grammatically incorrect. The verb "to rob" is followed by a direct object, but the phrase "is robbed all his possessions" is not a direct object.
• Someone has been robbed by the man of all his possessions is in the present perfect tense, while the original sentence is in the past tense.
• The man was being robbed of all his possessions by someone is in the past progressive tense, while the original sentence is in the simple past tense.