By definition, we have to:

The pedestrian is a person who, without being a driver of a vehicle, transits on foot through public spaces of the city.

We use this definition to complete the given sentence.

We have then:

El peatón es la persona que camina en la acera.

The translation of the sentence in English is:

The pedestrian is the person who walks on the sidewalk.

Answer:

c. el peatón

El peatón es la persona que camina en la acera.