Gives the mean time for which systemremained available for use.
a) mttf
b) mtbf
c) mttr
d) mttd

d)MTTD

Explanation: MTTD(Mean Time To Detect) is the time for the discovery of the  problem in a system.  It is basically the average time until the problem is noticed by someone. For e.g.- if there is a problem in a computer system at 9:00 am and the problem is noticed at 9:10 am by then user then the MTTD is  10 minutes . So, MTTD gives the mean time for which the system remains available for use.

answer; i believe the answer is tls,transport layer security;