Which chemical is specifically used to kill or repel birds?

The correct choice is C, avicide, because this term specifically refers to chemicals designed to kill or repel birds. Avicides are formulated to address issues related to bird populations, which can be a concern in various contexts such as agriculture, aviation, or public health.

In contrast, pesticides is a broad term encompassing substances used to manage pests, including insects, rodents, and fungi, but it does not specifically identify agents targeting birds. Herbicides are chemicals used to kill or inhibit plant growth, particularly unwanted weeds, while fungicides are designed to combat fungal infections in plants. Each of these terms has a specific application relevant to different types of organisms, which further differentiates avicides as the specialized category for addressing bird-related issues.