Question

how can humans start secondary succession? by plowing an extensive land area by letting livestock graze on grasslands by overwatering a crop field

Explanation:

Secondary succession occurs in areas where a disturbance has destroyed an existing ecosystem but left the soil intact. Plowing an extensive land area and letting livestock graze on grasslands are human activities that can create such disturbed areas. Overwatering a crop field mainly affects soil moisture and may not necessarily start secondary succession in the same sense as the first two activities.

Answer:

by plowing an extensive land area, by letting livestock graze on grasslands