The Chef is cooking dinner for the guest. 

Change the given sentence into passive voice.

When a verb's subject (in a sentence) also performs the action in the verb, the verb is said to be in the Active Voice. When the verb's subject (in the sentence) receives the action named in the verb, the verb is said to be in the Passive voice. 
According to the rule on changing active voice into passive, if a sentence is in present continuous tense (Helping verb "is") to convert into passive given sentence must change into present continuous tense (helping verb is + being). Therefore, option B is correct.