The correct answer is Studied
Step-by-step explanation
Research is the action by which humans deepen their knowledge of a specific topic by collecting correct information. Research is related to study since this activity is about the same action of acquiring knowledge and applying it. According to the above, the sentence when it refers to "investigated the bees" refers to Mikaila studying the issues related to bees to have a broader knowledge of them. According to the above, the correct answer is Studied