The empirical method of study is based on a. guesswork b. practice c. observation d. statistics

Observation                              

Explanation:

Empirical studies is observational evidence-based analysis. It refers to a way of obtaining insight through directly and indirectly evaluation or practice. Empiricism views more than most types of work. It is possible to mathematically or subjectively evaluate empirical proof.

Research design in such studies varies depending on the area and the problem being examined. Several scientists combine theoretical and practical types of study to adequately respond to comments that can not be examined in laboratory conditions, especially in humanities and learning.

Thus, from the above we can conclude that the correct option is C.

the first successful, urban blues recording, selling more than 75,000 copies, was made in year 1920.

mamie smith recorded the first commercially successful example of the urban blues,   with the recording of “crazy blues”."crazy blues" was recorded on august 10, 1920, in new york city. within a month of the release of “crazy blues” it had sold 75,000 copies.