Ectoparasites are a group of?

Ectoparasites are a group of organisms that feed on the exterior of their host. In the context of this question, the correct choice identifies nematodes that specifically inject a spear-like mouthpart into root tissue. This definition aligns with the behavior of certain ectoparasitic nematodes, which penetrate plant roots to extract nutrients, ultimately harming the plant. This type of feeding strategy directly illustrates how ectoparasites can negatively impact plant health by disrupting the root system and impeding nutrient and water absorption.

Other groups mentioned in the options, such as fungi, bacteria, and beneficial insects, do not fit the definition of ectoparasites. Fungi that thrive on dead organic matter serve different ecological roles, including decomposition. Bacteria that live in the soil can be beneficial or harmful but do not fall under the category of ectoparasites. Beneficial insects typically support plant health through pest control or pollination rather than feeding on the plants themselves. Therefore, the identification of nematodes with a specific parasitic behavior showcases a clear example of ectoparasitism, making it the correct choice.