Which of the following do herbicides kill?

Herbicides are specifically formulated to target and kill unwanted plants, primarily weeds. Weeds compete with cultivated plants for nutrients, light, and space, making them a significant concern in agriculture and gardening. Herbicides work by disrupting specific biological processes in plants, such as photosynthesis, cell division, or protein synthesis, which leads to the plants' death while posing minimal risk to the crops being grown.

The other options, while they represent other categories of unwanted organisms, are not affected by herbicides. Algae, fungi, and insects require different types of control measures, such as algicides, fungicides, and insecticides, respectively, each designed to address their unique biological structures and life processes. Therefore, the focus of herbicides is exclusively on plant life, particularly unwanted or invasive plant species.