What is a nematode?

A nematode is accurately described as a type of microscopic roundworm that resides in the soil. These organisms play significant roles in various ecosystems, especially in soil health and plant interactions. Nematodes can be either beneficial or harmful, but the distinguishing characteristic is their roundworm shape and microscopic size.

In soil environments, beneficial nematodes contribute to nutrient cycling and help control pest populations. They feed on bacteria, fungi, and even other pests, thus promoting a balanced ecosystem. On the other hand, certain species of nematodes can be plant parasites, leading to diseases and damage to crops. However, the definition of a nematode as a roundworm is primarily what makes option B the best choice in this context.

The other options detail aspects that do not correctly define nematodes—such as categorizing them as insects, confusing them with plant diseases, or misrepresenting their role in the soil without specifically recognizing their worm-like structure. This focus on the roundworm aspect and their soil-dwelling nature is key to understanding their significance in agriculture and ecology.