The correct answer is B. Las frutas frescas
Step-by-step explanation:
In Spanish, most plural nouns end in "s" or "-es" this means you can change a singular noun and make it plural by adding "s" or "-es" which varies according to the word. Indeed most word ending in vowels become plural if you add "-s" but you will need to add "-es" in case the word ends in a consonant or some accented vowels. Additionally, in a sentence besides making the noun plural, you will need to change to plural other sections of the sentence including the article, adjectives, and verb forms these sections need to agree in number.
In the case of the sentence "La fruta fresca" that means the fresh fruit all "la" that is an article for feminine nouns, "fruta" that means fruit, and "fresca" that is an adjective equivalent to fresh are in singular, you can know this because none of these words en in "s" or "-es". This implies you need to change all these words to plural, and as they all end in a vowel and do not have any accent all you need to do is to add "-s" to each word as "las frutas frescas".