Final answer:
A mixed base grease usually consists of two different metallic soaps, like sodium and calcium greases, leveraging the benefits of both types. The option (A) is correct.
Step-by-step explanation:
A mixed base grease is typically a combination of two different metallic soaps. In the options provided, the two bases that can be combined to form a mixed base grease are sodium (A) and calcium (D). Therefore, the correct answer would be a combination of both sodium and calcium soaps.
Notably, mixed base greases can offer the advantages of both types of soaps. Sodium greases have a high melting point and are water-resistant, while calcium greases are known for their good mechanical stability and water resistance at lower temperatures. The presence of sodium hydroxide and calcium hydroxide as the base components in greases promotes these properties. Therefore, option (A) is correct.