The Spanish word for radio is "la radio." In Spanish, nouns are either masculine or feminine and require the appropriate corresponding article. Radio is a singular feminine noun and requires the use of "la" in front of it.
Most Spanish nouns that end in "-o" are masculine nouns, but the word "radio" is an exception to this rule. Singular masculine nouns use "el" in front of them.
In cases where a word refers to a person or animal with a gender, the word varies based on the gender. For example, "gato" is the word for a male cat, while "gata" is used for a female cat. Other words are masculine or feminine based on Spanish language guidelines.