Final answer:
The most likely conclusion is that the virus is responsible for the necrotic ring spots.
Step-by-step explanation:
The most likely conclusion the farmer can make about the virus infecting his apple trees is option d: The virus is responsible for the necrotic ring spots. The presence of necrotic ring spots on the apple trees indicates that the virus is causing damage to the trees. The fact that only the trees with fungi on their trunks are affected suggests that the virus may be transmitted by the fungi. However, the exact mode of transmission cannot be determined based on the given information. It is important for the farmer to take necessary measures to prevent the spread of the virus and minimize the damage to his orchard.