Final answer:
The nurse should assess the client's pain level and emotional state to determine the cause of the crying.
Step-by-step explanation:
The nurse caring for the 35-year-old client who is 12 hours post mastectomy should respond by using analysis to determine the cause of the crying. Option C, which states 'Let's assess the client's pain level and emotional state to determine the cause of the crying,' is the best response. By assessing the client's pain level and emotional state, the nurse can gather information to identify the underlying reason for the client's tears and provide appropriate support and care.