Answer:
C
Step-by-step explanation:
[tex]\sqrt{3}\cdot\sqrt{21}= \\\\\sqrt{3\cdot 21}= \\\\\sqrt{63}= \\\\3\sqrt{7}[/tex]
Therefore, the correct answer is choice C. Hope this helps!
3√7
√3×√21=√63
√63=√7×√9=√7 × 3
=3√7
