<p><span>The correct answer is (</span><span class="ans">C) allows</span><span>.</span><br></p>
<p>In this sentence, the subject is "agricultural machinery" and it is being used in a general sense. The verb should agree with the subject in number. Since "agricultural machinery" is singular, the verb should also be singular. Therefore, options (A) and (D), which use the plural verb "allow", are incorrect.</p>
<p>(B) "which" uses the singular verb "is allowed", is also incorrect because it suggests that the machinery is being allowed by someone or something else, which is not the intended meaning of the sentence.</p>
<p>(C) "which" uses the singular verb "allows", correctly agrees with the subject "agricultural machinery" and conveys the intended meaning that the machinery enables or permits farmers to produce healthier crops and higher yields.</p>