Julia collected several valuable antiques over her lifetime. She set up an inter vivos trust to leave the antiques to her son, Edward. She also included a provision in the trust indenture stating that the trust's property would revert back to Julia at her discretion. This is an example of a:

a$)$ discretionary trust.

b$)$ revocable trust.

c$)$ mere power trust.

d$)$ precatory trust.