B is the correct answer. The person (farmer) is walking (wading) in water, when partially immersed in his bare feet.

"A" is incorrect because the farmer is wading, not his feet. ::::: By the way, your title should be "a grammar question," that is, a question regarding grammar. A "grammatical question" is just a question that is phrased in a grammatically correct way, which yours was, so the title is not technically "wrong," but I'm fairly sure it's not what you meant. ;)

I believe that A would be incorrect because wading should be wade in order to make sense.