beacuse of s.d is known (belongs to population),my reason is this term of u :
" If the standard deviation of individual blood pressure readings is known to be 11 millimeters of mercury"
so a 99% confidence interval for the true mean diastolic blood pressure of all people is:
L= 99 _ Z(.95)*11
U= 99 + Z(.95)*11
but if your intent in this term is not that s.d belongs to population
so instead of Z(.95), u must use t(.95) with 79 as degree of freedom.