Question 1194618
use combination:

{{{6C4=6!/((6-4)!4!)=6!/(2!4!)=6!/(2!4!)=(6*5*4*3*2*1)/((2*1)4*3*2*1)=(6*5)/(2*1)=3*5=15}}}

answer: 
b.
{{{15}}}