The method of data collection that covers the widest area is the mailed questionnaire method. This method involves mailing a set of questions to a specific group of people, who then fill out and return the questionnaire by mail.

Advantages of Mailed Questionnaire Method:

1. Wide Coverage: The mailed questionnaire method covers the widest area as it can reach a large number of people located in different geographical areas.

2. Convenient: This method is convenient for both the researcher and the respondent. The researcher can easily send the questionnaire to a large number of people without the need for a physical presence, and respondents can fill out the questionnaire at their own pace and convenience.

3. Cost-Effective: Mailed questionnaires are relatively inexpensive to produce and distribute compared to other methods of data collection such as direct interviews or telephone interviews.

4. Anonymity: Respondents are more likely to provide honest answers to sensitive questions when they are assured of anonymity.

Disadvantages of Mailed Questionnaire Method:

1. Low Response Rate: The mailed questionnaire method has a low response rate as many people may not respond to the questionnaire, leading to incomplete or biased data.

2. Delayed Responses: Responses to mailed questionnaires may take a long time to be received, leading to delayed data collection and analysis.

3. Limited Control: The researcher has limited control over the quality of responses as there is no opportunity to clarify or probe further into responses.

In conclusion, the mailed questionnaire method covers the widest area among the different methods of data collection. However, it has its own advantages and disadvantages, and researchers should carefully consider these factors when deciding which method to use.

Option b) is correct as widest area is covered with mailing the questionnaire as other methods take much time to cover more area but one is not sure about the response through mailed questionnaire method