Concept:Convert the total work into man-days and equate it with the work actually done by the changing number of men.Explanation:Since 100 men complete 31​ of the work in 10 days,total work =100×10×3=3000 man-days.Work done in first 10 days =100×10=1000 man-days.On the 11th day, there are 150 men, so work done =150 man-days.From the 12th to the 18th day, there are 200 men for 7 days.So, work done in these 7 days =200×7=1400 man-days.Total work done by the end of the 18th day =1000+150+1400=2550 man-days.Remaining work =3000−2550=450 man-days.Let x men be discharged at the end of the 18th day.Then the remaining men =200−x.Remaining working days =5.So, 5(200−x)=450.200−x=90.Hence, x=110.Answer:110 men must be discharged.Therefore, the correct option is C.