Answer:
D. 0.95
Step-by-step explanation:
The probability that at any moment, at least one of the systems will work is determined by the probability that system I works, added to the probability that system II works, and subtracted by the probability that both systems work:
[tex]P(S_I\cup S_{II}) =P(S_I)+P(S_{II})-P(S_I\cap S_{II})\\P(S_I\cup S_{II}) =0.85+0.90-0.80\\P(S_I\cup S_{II}) =0.95[/tex]
The probability that at any moment at least one of the systems will work is about 0.95.