<p>Let's analyze the sentence:<br></p><p><strong>The lack of rainfall ____ challenges for local farmers (who depend on river water and rainfall for irrigation).</strong></p><p dir="ltr"><span>The part wrapped with "()" is an adjective clause.</span></p><p dir="ltr"><span><span>This sentence fits the pattern: S + V + O. The subject is “The lack of rainfall”, the verb is the blank. The object is “challenges for local farmers.” In other words, the blank should be filled with a verb. </span></span></p><p><strong>Subject-verb agreement rule:</strong></p><p></p><div>All the four choices are verbs.</div><div><br></div><div>The subject “The lack of rainfall” is singular and should go with a singular verb. (A) "is posting" is a singular verb, so (A) is the correct answer. <br></div><div><br></div><div>(C) "have posed" is incorrect because the subject “The lack of rainfall” is singular and should go with a singular verb, but “have posed” is a plural verb. </div><div><br></div><div>(B) "is posed" and (D) "has been posed" are incorrect because based on the meaning of the context, the lack of rainfall actively poses challenges for people instead of being posed challenges.<br></div><p></p>