Final answer:
The force that keeps you from sliding on an icy sidewalk is static friction.
Step-by-step explanation:
The force that keeps you from sliding on an icy sidewalk is static friction (option a). Static friction is the force that acts between two objects in contact with each other and prevents them from sliding against each other when there is no relative motion between them.
For example, when you're standing on an icy sidewalk, static friction between the soles of your shoes and the icy surface keeps you from sliding.