Question

optimum means :
not fit to assume responsibility
strikingly unconventional
occurring or appearing at irregular intervals
most desirable possible under a restriction

Explanation:

The word "optimum" is related to the best or most favorable condition. "Most desirable possible under a restriction" aligns with this meaning. "Not fit to assume responsibility" is about capability, "strikingly unconventional" is about being non - traditional, and "occurring or appearing at irregular intervals" is about frequency.

Answer:

most desirable possible under a restriction