Right answer:
a. la acera
Acera is a feminine and singular noun that translates into sidewalk ans is a a paved path for pedestrians at the side of a road. We use the definite article la because in Spanish definite articles must agree in both gender and number with the nouns they introduce. Therefore:
IN SPANISH:
Los peatones caminan en la acera
IN ENGLISH:
Pedestrians walk on the sidewalk
