Given: (x+3)6 Here, n = 6 ∵ n = 6 and it as even number As we know that, in the expansion of (a+b)n the middle term is (
n
2
+1)th term if n is even. So, (
6
2
+1)th=4 th term is the middle term in the expansion of (x+3)6 As we know that, the general term is given by: Tr+1=nCr.an−r.br Here, n=6,r=3,a=x and b=3 T4=T(3+1)=6C3.x3.(3)3=540x3 Hence, option D is the correct answer.