Question 1020885
[{{{19}}}, {{{25}}}, {{{3}}}]-> common rule:{{{(25-19)/2=6/2=3}}}
[{{{4}}}, {{{26}}}, {{{11}}}] ->{{{(26-4)/2=22/2=11}}}
[{{{x}}}, {{{51}}}, {{{9}}}]->{{{(51-x)/2=9}}}->{{{51-x=18}}}->{{{51-18=x}}}->{{{x=33}}}

[{{{33}}}, {{{51}}}, {{{9}}}]

so, answer is:
C {{{33}}}