The correct answer is A. Questionnaire.
A questionnaire is a list of questions that are asked to a group of people in order to collect data. It is a convenient and efficient way to collect data from a large number of people. In a predominantly illiterate area, a questionnaire is the best option because it can be administered by a trained interviewer who can read the questions to the respondents and record their answers.
A schedule is a list of questions that are asked to a group of people in order to collect data. It is similar to a questionnaire, but it is usually more detailed and requires more time to complete. A schedule is not a good option for a predominantly illiterate area because it would be difficult for the respondents to understand the questions and complete the schedule on their own.
An interview is a conversation between an interviewer and a respondent. It is a good way to collect data that is not easily quantifiable, such as opinions and attitudes. However, interviews are time-consuming and expensive to conduct. In a predominantly illiterate area, interviews would be difficult to conduct because the interviewer would need to be able to speak the local language and understand the local culture.
Therefore, the best option for collecting data from a predominantly illiterate area is a questionnaire.