A. After the study of lizards, the naturalist planned to travel to remote lizard habitats.
Explanation:
In this case, after the study of lizards, the speaker planned something. What he planned is modified by another verb. He planned <em>"to travel to remote lizard habitats.</em>"
So, in this case, we have an infinitive phrase where <em>"to travel" </em>is modifying the verb <em>"to plan."</em>