Answer: C. Increase the sample size
Step-by-step explanation: The confidence interval is a interval estimate of two numbers. From the question, the width confidence interval
Can be modified into a more narrow margin by reducing the standard error. The standard error is the value obtained from the division of standard deviation and the square root of sample size. Hence, a decrease in the standard error will give a more narrow confidence interval estimate. The standard error can be reduced by increasing the denominator of the standard error formula which is the sample size.