What can be a simple effect of upwelling related to convection current?

Correct answer: Spreading at the surface

Upwelling means that material from a deeper, hotter part of the mantle rises toward the surface. When such movement occurs beneath oceanic plates, it can supply material that cools and becomes new crust. Continued movement away from the rising zone can produce spreading at the surface, especially along mid-oceanic ridges. This is a simple way to connect convection with sea-floor spreading.

Option A, spreading at the surface, is therefore the best answer. Harvesting crops, a change in cloud colour, and a decrease in population are not direct geological effects of mantle upwelling. The question asks for a simple possible effect, not a complete description of every process. Rising material followed by outward movement gives the key link: upwelling can support surface spreading.