<p style="">The correct answer is (B) inform.</p><p style="">Here's the explanation:</p><p style="">This sentence is an imperative sentence, giving a directive or an instruction about what should be done when a sample is sent to clients. In imperative sentences, the subject (you) is often implied, and the verb is in the base form.</p><p style="">(A) "to inform" is an infinitive form of the verb and doesn't fit the structure of an imperative sentence.</p><p style="">(B) "inform" is the base form of the verb and correctly completes the directive, making the sentence read as an instruction: "When sending a requested sample to clients, inform them that [the] package is on its way."</p><p style="">(C) "informing" is a gerund or verb form acting as a noun and doesn't fit the structure of an imperative sentence.</p><p style="">(D) "informs" is the third person singular present tense of the verb and doesn't fit because the sentence is not making a statement about a habitual action or a fact.</p>