<p><span style="color:#27ae60;">Solving the Problem</span></p>
<p><strong>Method 1 (Algebra)</strong></p>
<p>We can start out by re-writing some of the root identities:</p>
<p>$(\sqrt{4x} + \sqrt{9x})^2 = (\sqrt{4}\sqrt{x} + \sqrt{9}\sqrt{x})^2$</p>
<p>This would then become</p>
<ul>
<li>$(\sqrt{4}\sqrt{x} + \sqrt{9}\sqrt{x})^2$</li>
<li>$(2\sqrt{x} + 3\sqrt{x})^2$</li>
</ul>
<p>We can now expand this out:</p>
<ul>
<li>$(2\sqrt{x} + 3\sqrt{x})(2\sqrt{x} + 3\sqrt{x})$</li>
<li>$2\sqrt{x}(2\sqrt{x})+2\sqrt{x}(3\sqrt{x})+3\sqrt{x}(2\sqrt{x})+3\sqrt{x}(3\sqrt{x})$</li>
</ul>
<p>And now we can simplify:</p>
<p>$4x+6x+6x+9x=25x$</p>
<p><strong>Method 2 (Choosing Numbers)</strong></p>
<p>Since we are told that $x>0$, let's choose $x=1$.</p>
<p>Now, we just plug in $1$ into the equation wherever there is an x:</p>
<ul>
<li>$(\sqrt{4(1)} + \sqrt{9(1)})^2$</li>
<li>$(\sqrt{4}+\sqrt{9})^2$</li>
<li>$(|\sqrt{4}|+|\sqrt{9}|)^2$</li>
<li>$(2+3)^2$</li>
<li>$5^2=25$</li>
</ul>
<p>Now, we can see that only when we put $x=1$ into the equation $25x$, we get the result of $25$.</p>
<p>So, the answer would be <span style="color:#27ae60;">D</span>.</p>