Which type of plant relies on animal vectors for pollination?

Cucumber is a type of plant that relies on animal vectors for pollination, particularly insects such as bees and butterflies. These animals are attracted to the flowers of the cucumber plant, which produce nectar and have bright colors, making them visible and appealing to pollinators. When these insects visit the flowers to collect nectar, they inadvertently transfer pollen from one flower to another, facilitating the reproductive process necessary for the production of fruits and seeds.

In contrast, grains like wheat, rice, and barley are primarily pollinated by the wind. These plants have evolved to produce a large amount of lightweight pollen that can be readily carried by air currents. They do not have the same need for animal interactions for pollination, as their reproductive strategy is geared towards wind dispersal rather than attracting animals. Therefore, the cucumber's reliance on animal vectors distinguishes it from the other options listed.