Final answer:
The minimum blow back on a safety valve of 100lbs is zero.
Step-by-step explanation:
A safety valve is designed to release excess pressure from a system to prevent damage or accidents. The blow back refers to the pressure that is exerted on the valve when it opens. In this case, the minimum blow back on a safety valve of 100lbs can be determined by considering the pressure at which the valve opens. If the valve opens at a pressure of 100lbs, then the blow back would be zero since the pressure is already equal to the set point. Therefore, none of the options provided (a) 6, (b) 2, (c) 4 are correct as the minimum blow back is zero.