Answer:
a. A reward was given to him by the king.
Step-by-step explanation:
To change any given sentence into the passive voice, the direct object in the active voice will be placed in the position of the subject while the subject in the active voice will be used at the start of the passive voice sentence. Moreover, the tense of the verb will also be changed accordingly.
Now, the given sentence "the king gave him a reward" is in the active form. So, to change it into the passive voice, the object "reward" will come at the start of the sentence while the subject "the king" will move to the end of the sentence.
Therefore, the new sentence will be-
A reward was given to him by the king.