ANS:A - machinability

Lead is added to 60:40 brass primarily to improve machinability. Brass is an alloy of copper and zinc, and the addition of lead to brass alloys can enhance their machinability. Lead acts as a lubricant during machining operations, such as cutting, drilling, and tapping, by forming a thin film of lead particles on the cutting tool and workpiece interface. This reduces friction and cutting forces, resulting in smoother cuts, better surface finish, and longer tool life. While leaded brasses have improved machinability, it's important to note that the addition of lead may have some adverse effects on other properties of the alloy, such as its strength and corrosion resistance. Therefore, the amount of lead added needs to be carefully controlled to achieve the desired balance between machinability and other mechanical properties. So, the correct answer is: machinability.