Select the option that expresses the given sentence in active voice.

The deer was killed by the boar.

The correct answer is: The boar killed the deer.

To convert a sentence from the passive voice to the active voice, we can follow these steps:
1. Identify the parts of the sentence: In the passive sentence, "The deer was killed by the boar", the subject is "The deer" (the receiver of the action), the verb phrase is "was killed" (in the simple past passive voice), and the agent is "the boar" (the doer of the action, introduced by "by").
2. Make the agent the new subject: To make the sentence active, we place the doer of the action, "The boar", at the beginning of the sentence as the subject.
3. Convert the verb to the active voice: The passive verb phrase "was killed" (simple past passive) is changed to its active counterpart in the simple past tense, which is "killed".
4. Make the receiver the object: The original passive subject, "the deer", is moved to the position of the direct object after the verb.
Combining these elements, we get the active sentence: "The boar killed the deer."