The correct active form of the given sentence is Her behavior vexes me.

The passive voice of the sentence is I am vexed by her behavior. This sentence is in the present tense, which is used to describe an action that is happening now or that is happening repeatedly. The passive voice of the sentence is formed by putting the subject of the active sentence after the verb "to be" and the past participle of the main verb. In this case, the subject of the active sentence is "I" and the main verb is "vex." The past participle of "vex" is "vexed." So, the passive form of the sentence is I am vexed by her behavior.

The active voice of the sentence is formed by putting the subject of the passive sentence before the verb and removing the verb "to be." In this case, the subject of the passive sentence is "her behavior" and the main verb is "vex." So, the active form of the sentence is Her behavior vexes me.

The other options are incorrect because:

• Her behavior has vexed me. is in the past tense, while the original sentence is in the present tense.
• Her behavior vexed me. is the past tense form of the active voice of the original sentence.
• Her behavior vexes me. is the present tense form of the active voice of the original sentence.
• Her behavior is vexing to me. is a phrase that describes how the speaker feels about the behavior, but it is not a complete sentence.