A. $\sqrt{9}+\sqrt{16}=\sqrt{25}$
$\to$ Sai
$\sqrt{9}+\sqrt{16}=3+4 = 7 \ne 5$
B. $\sqrt{(-5)^2}=25$
$\to$ Sai
$\sqrt{(-5)^2}=|-5|=5$
C. $\sqrt{169}-\sqrt{144}=\sqrt{25}$
$\to$ Sai
$\sqrt{169}-\sqrt{144}=13-12=1 \ne 5$
D. $\sqrt{9}.\sqrt 4 = \sqrt{36}$
$\to$ Đúng
$\sqrt{9}.\sqrt 4 = 3.2 = 6$