Hello ^-^
The well defined set in the following is Letter C. A set of english names of the days in a week.
The days in a week in english names are Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, and Sunday. It consists of 7 days in a week.
In Letter A, we can't defined the teachers names or all teachers are good.
In Letter B, we didn't know who are they or who are the speakers.
In Letter D, we can't recognize who are the family names that start in letter "A".
So, the final answer is Letter C.
In defining well defined set, choose the most appropriate, recognized, defined, and know by the people.
Hope It Help
^-^
