Answer: c. 6.14 and 6.86
Explanation:
The confidence interval for population mean is given by :-
, where n= sample size
= Sample mean
= Population standard deviation
z*= Critical z-value.
As per given , we have
n= 85
For 0.95 degree of confidence interval , the critical z-value is 1.96. (By z-table).
So , the required confidence interval will be:

Hence, the confidence interval for the population mean is c. 6.14 and 6.8 .