The correct answer is C. have been picking/placed.
The sentence is in the present perfect progressive tense, which is used to describe an action that started in the past and continues to the present. In this case, the workers have been picking peaches since some point in the past and are still picking them now. The other workers are placing the peaches in wooden boxes.
Option A is incorrect because the present progressive tense is used to describe an action that is happening at the moment of speaking. In this case, the workers are not picking peaches at the moment of speaking, so the present progressive tense is not appropriate.
Option B is incorrect because the past progressive tense is used to describe an action that was happening at a specific time in the past. In this case, the workers are not picking peaches at a specific time in the past, so the past progressive tense is not appropriate.
Option D is incorrect because the simple past tense is used to describe an action that happened at a specific time in the past. In this case, the workers are not picking peaches at a specific time in the past, so the simple past tense
is not appropriate.Option E is incorrect because the past perfect progressive tense is used to describe an action that started and ended in the past. In this case, the workers are still picking peaches, so the past perfect progressive tense is not appropriate.