The answer is C.
Given that, the mean of the observations x,x+3,x+5,x+7 and x+10 is 9.
∴x+x+3+x+5+x+7+x+105=9
⇒5x+25=45
⇒5x=20
⇒x=4
Last three observations are x+5=4+5=9,x+7=4+7=11
And x+10=4+10=14
So, the mean of last three observations =9+11+143=343=1113
Hence, the mean of last three observations is 1113.