The correct option is C 10C4
When x1<x2<x3<x4<x5<x6
(Since we have to form 6 digit number, ∴x1≠0)
Numbers can be selected in 9C6 ways and can be arranged in 1 way.
When x1<x2<x3=x4<x5<x6
Numbers can be selected in 9C5 ways and can be arranged in 1 way.
∴ required ways
=9C6+9C5=10C6=10C4
[∵ nCr−1+ nCr= n+1Cr]