18. Which of the equilibrium types allows the ratio of daughter to parent activity
remains fixed?
(a) Transient Equilibrium
(b) Secular Equilibrium
(C) No Equilibrium
(d) Chain Equilibrium

Transient Equilibrium

Explanation:

[source; wikipedia]

In nuclear physics, transient equilibrium is a situation in which equilibrium is reached by a parent-daughter radioactive isotope pair where the half-life of the daughter is shorter than the half-life of the parent. Contrary to secular equilibrium, the half-life of the daughter is not negligible compared to parent's half-life. An example of this is a molybdenum-99 generator producing technetium-99 for nuclear medicine diagnostic procedures. Such a generator is sometimes called a cow because the daughter product, in this case technetium-99, is milked at regular intervals.[1] Transient equilibrium occurs after four half-lives, on average.[2]

(i hope this sort of helps? :"] )