D. 1 p.m.
Feedback The three machines 1 hour’s work = 1/8 + 1/10 + 1/12 = 37/120
Their, work from 9 a.m. to 11 a.m. = 2 hour’s work = 37/120 × 2= 27/60
The remaining work = 23/60
Machines Q and R’s 1 hours work = 1/10 + 1/12 = 11/60
Therefore, the time taken by Q and R to finish the remaining 23/60 work = 23/60/11/60
= 23/60 × 60/11 = 23/11 = 2 hours approx.
Therefore, the approximate time at which the work is finished = 11 + 2 = 1 p.m.