In what scenario must an arborist follow federal regulations related to pesticides?

An arborist must follow federal regulations related to pesticides when applying pesticides to retard decay or disease. This is crucial because pesticides are subject to strict regulations under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) to ensure that they are used safely and effectively. These regulations cover not only the types of pesticides that can be used but also stipulate the label instructions, safe handling procedures, and the proper application methods. Non-compliance can lead to legal repercussions and harm to the environment and public health.

In contrast, applying for a job, training apprentices, and conducting educational workshops do not directly involve the use of pesticides, so the same level of regulatory oversight does not apply. Federal pesticide regulations are specifically targeted towards activities involving the application of these chemical substances, making option C the clear scenario wherein compliance is necessary.