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.

since it can be assumed that what aslam is making is a template by compiling all the commonly used excel formatting in several worksheets in an excel document, aslam can choose to save it using the .xltx or .xltm format instead. thus, aslam can distribute the templates he has made to his team members in an easier manner.

you, i'm trying to get a higher rank, this really ! : )