Coeff. of $x^{50}$ in $(1+x)^{1000}$ + $x(1+x)^{999}$ + $x^2(1+x)^{998}$ + .... + $x^{1000}$ is

Question

$\begin{array}{1 1} ^{1000} C_50 \\ ^{1001}C_{50} \\ ^{1001}C_{51} \\ ^{1000}C_{51} \end{array}$