It means the range of a variable, centered at the most likely mean value, that the variable is in 95% of the time. The range that the variable is 99% of the time is wider. Therefore the answer is (b)
So the following would be b as well:
For two normal distributions with the same mean, but different standard deviations, the 95% confidence interval will be:
(a) The same for both distributions
(b) Larger for the distribution with the smaller standard deviation
(c) Smaller for the distribution with the smaller standard deviation
95% confidence interval is approximately 2 standard deviations (SDs) away from the mean in each direction (}2 SD). Do you still believe that the 95% confidence interval would be larger for a smaller SD?
I hope this helps. Thanks for asking.