We need to find the coefficient of x7 in the expansion (1−x2+x3)(1+x)10 Now,(1+x)10 will have all the powers of x from 0 to 10. Multiplying these powers by 1,x2 and x3 will yield different results but we are interested in finding only the coefficient of x7 When we multiply x7 of (1+x)10 by 1,x5 of (1+x)10 by x2 and x4 of (1+x)10 by x3.we will get x7 coefficient of x7 in (1+x)10 is 10C7=120, coefficient of x5 in (1+x)10 is 10C5=252, coefficient of x4 in (1+x)10 is 10C4=210 adding 120 and 210 and subtracting(since x2 has a negative sign) 252 we get coefficient of x7 as 78 Therefore our answer is option 'B'