A sentence is in active or passive voice depending on whether its verb is in active or passive voice. A verb is in active voice when its form shows that the Subject does something/ some action. A verb is in passive voice when something is done to the Subject. In passive voice, the doer of the action may appear in the phrase, "by .....". This could be one easy way of identifying a sentence in passive voice. However, in some cases, this phrase is omitted.
Let us look at the subject of each sentence.
In A), "Many people" is the subject who do the action "praised". Hence, this sentence is in active voice and not the correct answer.
In B), "Robinhood" is the subject who "did" heroic deeds. Hence, this sentence is in active voice and not the correct answer.
In C), "Robinhood" is the subject, who "was praised" i.e an action was performed on the subject. Note the use of the phrase, "by many people" indicating "many people" as the doer of the action word "praised". Hence, this sentence is in passive voice and is the correct answer.
In D), "Robinhood" is the subject who "received" a lot of praise. The action word or verb here is "received", which was performed by the subject. Hence, this sentence is in active voice and not the correct answer.
Given the above, option C is the correct answer.