<p>It’s the number of states that is growing. Choice C has the adverb “growingly” which would modify the word adopted, which suggests that it’s the act of adoption that’s growing and not the number of states?</p>
<p>“growingly” as used in choice C could mean that laws prohibitng e-waste are starting to “grow” in certain states, whereas “a growing number” refers to an <em>amount</em> of states that are increasing.</p>