To factor the expression \(100x + 1000\) completely, we can first factor out the greatest common factor (GCF), which is \(100\):
\[ 100x + 1000 = 100(x + 10) \]
Thus, the completely factored form of the expression is:
\[ 100(x + 10) \]
So, the correct response is:
100(x+10)