hypothesis test, confidence interval (postal employees)

An SRS of 100 postal employees found that the average time these employees had worked for the postal service was 7 years with standard deviation 2 years. Assume the distribution of the time the population of employees have worked for the postal service is approximately normal with mean mew. Are these data evidence that mu has changed from the value of 7.5 years of 20 years ago? To make this determination we test the hypotheses:

Ho: mu equal to 7.5
Ha: mu not equal to 7.5

What I'm having trouble with is the following:

A 95% confidence level for the mean time, mu, that the population of postal service employees have spent with the postal service is:

A) 7 ± 2
B) 7 ± 1.984
C) 7 ± 0.2
D) 7 ± 0.3968

Thanks a bunch!

I hope no one helps you. You have merely copied and pasted a question. You have provided nothing to help us understand what it is that is confusing to you.

No sensitivity points for that comment, but it is a good point.

This problem is intended to test your knowledge of the standard error of a sample. What clues have you?