For infection to occur, which of the following methods of penetration is necessary?

For infection to occur, direct penetration is necessary as it refers to the process where pathogens, such as fungi, viruses, or bacteria, invade host tissues by breaking through the outer protective layers, like the epidermis or cell walls. This method is essential because it allows the pathogen to access the nutrient-rich environment within the plant tissues, which is crucial for its survival and reproduction. Direct penetration often involves specialized structures or enzymes that help the pathogen breach the plant's defenses.

Other options like airborne spores and soil contact can also play roles in the spread and establishment of infections, but they do not directly facilitate the penetration into the host. Airborne spores may allow the pathogen to reach potential host plants, while soil contact may provide a pathway for root infection, but neither method guarantees the direct invasion that is necessary for a successful infection. Photosynthesis is unrelated to pathogen entry and primarily relates to plants' ability to convert light into energy for growth.