<p>Several countries discussed how to <strong>strengthen</strong> their cooperation on the prevention and control of certain communicable diseases.</p>
<p>The verb "strengthen" is the correct choice in this sentence. Here's the explanation:</p>
<p>The sentence is talking about countries discussing how to improve or enhance their cooperation on the prevention and control of certain communicable diseases. The word "strengthen" means to make something stronger or more effective. In this context, it means that the countries want to make their cooperation on disease prevention and control more robust and efficient.</p>
<p>The other options are not suitable:</p>
<p>(A) "Introduce" means to bring something new into a situation. In this context, it doesn't make sense to introduce cooperation because the countries are already discussing it. </p>
<p>(B) "Charge" means to demand payment or assign responsibility. It is not appropriate in the context of discussing cooperation.</p>
<p>(D) "Substitute" means to replace one thing with another. In this context, it doesn't make sense to substitute cooperation because the countries want to improve their existing cooperation, not replace it.</p>