Thiocarbamates are primarily used to achieve which of the following?

Thiocarbamates are primarily utilized for their effectiveness as herbicides that target grassy weeds. Their primary mode of action is through the inhibition of seed germination and growth in certain types of plants, particularly grasses. By inhibiting germination, thiocarbamates help in controlling unwanted grassy species in crops, contributing to overall weed management strategies.

The option concerning the prevention of rapid evaporation from the soil surface does not accurately reflect the primary use of thiocarbamates. Instead, they focus on weed control. In contrast, the other options relate to functions that are not associated with thiocarbamates. Inhibiting photosynthesis pertains more to herbicides that act on different pathways; acting as synthetic hormones is characteristic of other classes of growth regulators, and while some pesticides can slightly affect evaporation indirectly, it is not a primary function of thiocarbamates.

Thus, the correct answer is that thiocarbamates are specifically intended for controlling grasses, aligning with their mode of action in managing unwanted plants in agricultural settings.