Final answer:
The nurse aide should place the thermometer under the tongue to take a client's oral temperature. Lubricant is not necessary for this method. The correct option is C.
Step-by-step explanation:
The nurse aide should place the thermometer under the tongue to take a client's oral temperature. This method is commonly used and provides an accurate reading. Other options like placing the thermometer in the rectum or under the arm are used for different types of temperature measurements.
Putting lubricant on the thermometer is not necessary for taking an oral temperature as it may interfere with the accuracy of the reading. Hence, C is the correct option.