Which of the following is a pest that affects hop plants?

The hop aphid is a known pest that specifically targets hop plants, making it the correct response. This small insect often infests hop plants, feeding on the sap and weakening the plants. Its feeding can lead to a range of negative effects, including reduced hop yield and the potential transmission of plant viruses. The presence of hop aphids can be detrimental not only to the health of the plants but also to the quality of hops produced for brewing.

In contrast, the other pests listed do not primarily affect hop plants. The potato beetle is associated with potato crops, while leafhoppers are more commonly known to target a variety of crops but do not have a significant impact specifically on hops. Root maggots can affect various vegetables and plant roots but, again, are not a common pest for hop plants. Understanding the specific pests that affect crops is crucial for effective management and cultivation practices in hop farming.