Human beings can see the spatial relations among objects by processing

The answer is option A.

Premise 1: Human beings can see the spatial relations among objects by processing information conveyed by light.
Premise 2: Scientists trying to build computers that can detect spatial relations by the same kind of process have so far designed and built stationary machines.
Conclusion: These scientists will not achieve their goal until they produce such a machine that can move around in its environment.

Necessary Assumption: Only machines capable of moving around can detect spatial relations among objects.

Any answer choice that supports the conclusion that the scientists will not achieve their goal with their current design of machines that are stationary and reinforces the assumption that the detection of spatial relations among objects is only possible for a body capable of motion or for a body in motion will strengthen the argument.

Option A states that Human beings are dependent on visual cues from motion in order to detect spatial relations. This means that the ability of humans to detect spatial relations among bodies hinges on their capability to move, since humans depend on visual cues from motion. This is in line with the argument and the necessary assumption stated above.

The only contender for option A is option B. Option B states that Human beings can often easily detect the spatial relations among objects, even when those objects are in motion. Although option B talks about motion, it rather focuses on the motion of the spatial bodies. But the idea of the argument is not for the spatial bodies themselves to move, but the device or humans who are to detect the spatial relations must rely on their motion. Hence, although B talks about motion, it focuses on the motion of the spatial body themselves without mentioning the need for the detecting body or system to rely on their ability to move in order to detect the spatial relations.