Which 54th move could have been used by Edgar Walther (White) of the Schachgesellschaft Zürich to present Bobby Fischer with unresolvable problems?
a) 54. a4
b) 54. a3
c) 54. Kb6
d) 54. b4
The picture below shows the position after Blacks 53rd move:
wKb5,Bf3,Pa2,b2,h3/bKd7,Bd6, Ph4