Final answer:
Calcium base greases are the cheapest and water-insoluble semi-solid lubricants, preferred for their cost-effectiveness and resistance to water.
Step-by-step explanation:
The question concerns which semi-solid lubricant is the cheapest and insoluble in water. Among the options provided, the cheapest and water-insoluble semi-solid lubricant is typically D) calcium base greases. These greases are generally used for a variety of applications due to their cost-effectiveness and resistance to water. They are made with calcium soaps which provide good protection against moisture, making them insoluble in water. This is similar to the substances listed, such as BaSO4 and AgCl, which are also insoluble in water, even though they do not serve as lubricants. Calcium base greases, however, are specifically designed for lubricating purposes where water insolubility is a valued property.