Each species of yucca plant is pollinated exclusively by one
species of yucca moth. this insect cannot complete its life cycle
in any other plant, as its larvae can only grow in the yucca
flower. which of these terms best describes the relationship
between the moth and the plant?

Relationship between Yucca plant and Yucca moth is OBLIGATE MUTUALISM.

Explanation:

Obligate mutualism- The mutual relationship between two partners where both partners are benefited by each other and can not survive without each other.

In case of Yucca plant and Yucca moth,they cannot survive without each other.Yucca moth larvae feed on seeds of Yucca plant and Yucca plant have a single pollinator i;e, Yucca moth. Yucca moth collects pollen from anther and fly to another flower and transfer that pollen to other flowers stigma. In this way they transfer the pollen , usually they make a hole in the ovary to lay eggs and directly transfer pollen to the inside of the stigma.

After laying eggs they mark it with some special kind of pheromone which alerts other moths not to lay egg inside the same flower or lay some eggs not much. So that they can be protected inside the flower and can be developed well.