A farmer observed that a certain crop plant produces seeds only when pollen from another plant of the same species is used. This type of pollination is best described as

Cross-pollination occurs when pollen is transferred from the anther of one flower to the stigma of a flower on a different plant of the same species. The farmer's observation that seeds form only when pollen comes from another plant indicates the plant is self-incompatible and requires cross-pollination. Wind and insect pollination describe the agents, not the type of pollination.