Final answer:
Plant viruses can be transmitted by insects through viruses on their mouthparts, those circulating in their bodies without multiplying, or viruses that multiply within the insect's body. All three methods are correct pathways for transmission.
Step-by-step explanation:
Transmission of plant viruses by insects can occur in multiple ways:
- Viruses associated with insect mouthparts: These may be transferred to plants when an insect feeds.
- Viruses circulating but not multiplying in the insect's body: Also known as mechanical transmission, where the virus is carried on the insect's body.
- Viruses that multiply within the body of the insect: In this case, the insect acts as a biological vector and can spread the virus over a longer period.
Therefore, the correct answer to the question is D) All of the above.