Rationale: Cefazolin is a cephalosporin, which has a cross-sensitivity with penicillin. Therefore, the nurse should ask the client about any previous reactions to cephalosporins before administering the medication. This can help identify clients who are at risk of developing an allergic reaction to cefazolin.

B) is incorrect because administering an antihistamine is not a preventive measure for an allergic reaction, but rather a treatment for mild symptoms. Antihistamines do not prevent anaphylaxis, which is a life-threatening reaction that requires immediate intervention.

C) is incorrect because monitoring the client's vital signs for 15 minutes after the injection is not enough to detect an allergic reaction, which can occur up to 72 hours after exposure. The nurse should monitor the client for signs and symptoms of an allergic reaction throughout the course of therapy.

D) is incorrect because diluting the medication with normal saline does not reduce the risk of an allergic reaction, but rather the risk of phlebitis, which is inflammation of the vein. Phlebitis is a common adverse reaction to cefazolin, but it is not related to allergy.