B

- Hướng dẫn giải

Chọn B

$\begin{array}{l}\lim 2n\left(\sqrt{n^2+1}-\sqrt{n^2-3}\right)=\lim 2n\frac{n^2+1-n^2+3}{\sqrt{n^2+1}+\sqrt{n^2-3}}\\ =\lim \frac{8n}{\sqrt{n^2+1}+\sqrt{n^2-3}}=\lim \frac{8n}{n\sqrt{1+\frac{1}{n^2}}+n\sqrt{1-\frac{3}{n^2}}}=\lim \frac{8}{\sqrt{1+\frac{1}{n^2}}+\sqrt{1-\frac{3}{n^2}}} =4\end{array}$