Answer: A. Either Mavis or Cameron is responsible for answering the phone.
In option A, the inclusion of the word "either" means that only one person is responsible, not both. This means that the verb refers to a single entity, which makes this the correct sentence.
In option B, "twenty years" is an expression of time that is taken as a single measure, and is therefore considered a single entity. The verb needs to be in its singular form.
In option C, the verb is not referring to "people," who are separate, plural entities. Instead, it refers to "number," and therefore, the verb needs to be expressed in its singular form.
In option C, the inclusion of the word "both" means that the verb refers to the two subjects simultaneously. Therefore, the verb needs to be written in its plural form.