What is the voice of the verb in the sentence: "An agreement was made by the leaders of the two countries."?
It is a sentence in Passive Voice. See explanation.
This sentence is in Passive Voice, because the subject is not the doer (leaders) but the thing which was done (an agreement).
In the Active Voice sentence the subject would be the leaders. The sample sentence could be:
The leaders of the two countries made an agreement.
If you look at the sentence in Passive Voice you can see that its verb has a form of to be + Past Participle