<p><span style="color:#27ae60;"><span style="font-size:20px;">Solving the Problem</span></span></p>
<p>This problem is easily solvable by just knowing the prime numbers up to 25:</p>
<p>$2, 3, 5, 7, 11, 13, 17, 19, 23$</p>
<p>We can see that:</p>
<ul>
<li>$c$ = the smallest prime greater than $21$ -> $23$</li>
<li>$d$ = the largest prime less than $16$ -> $13$</li>
<li>$cd$ = the product of $c$ and $d$ = $13 * 23 = 299$</li>
</ul>
<p>Therefore, the answer should be <span style="color:#27ae60;">A</span>.</p>