A signal processor software is expected to operate for 91.25 days after repair, and the mean software repair time is expected to be 5 minutes. Then, the availability of the software is:

Explanation

Availability = MTBF / (MTBF + MTTR). Here the mean time between failures is 91.25 days = 91.25 × 24 × 60 = 1,31,400 minutes and the mean time to repair is 5 minutes. So availability = 1,31,400 / (1,31,400 + 5) = 1,31,400 / 1,31,405 = 0.9999619, that is 99.9962%. Only 5 minutes of downtime against roughly three months of operation must give a figure just under 100%, so option (4) is the answer.