To simplify \(3\sqrt{21} + 7\sqrt{21}\), you can combine the like terms:
\[ 3\sqrt{21} + 7\sqrt{21} = (3 + 7)\sqrt{21} = 10\sqrt{21} \]
So the answer is:
\[ 10\sqrt{21} \]
Among the options provided, it corresponds to:
\[ 10\sqrt{21} \]
Therefore, the correct choice is:
10√21