The correct answer is A. Each journalist has to decide what beat they want to cover.
Step-by-step explanation:
In grammar, a pronoun refers to a word that replaces a noun, for example, "boy" can be replaced by the pronoun "he" or related words such as "his". In sentences, it is common nouns or proper names are not repeated over and over but pronouns are used to refer to the noun or antecedent. In this context, the antecedent and pronoun replacing it must agree in number and gender. In the case of "Each journalist has to decide what beat they want to cover" there is a pronoun-antecedent disagreement error because the pronoun used to replace "each journalist" is "they"; however, "journalist" is singular and should be replaced by "she or he wants to cover".