Question

She  could  ... walk .                                         a) hard     b) hardly     c) good

Answer 1

She  could   hardly  walk .  Она едва могла ходить.                                     
 a) hard   
  b) hardly    
 c) good

Answer 2

The correct answer would be:

c) good

The sentence "She could walk" implies that she has the ability to walk, and the word "good" in this context suggests that she is capable of walking well or proficiently.