In the context of global economic development, which scenario best illustrates the concept of "conditional convergence"?

a$)$ A developing country rapidly grows its economy after adopting policies and technologies similar to those used in more developed countries, gradually closing the income gap.

b$)$ A wealthy country experiences a significant economic downturn, leading to reduced income disparities with developing countries.

c$)$ An international organization provides unconditional financial aid to underdeveloped nations, leading to varied economic outcomes.

d$)$ Developed nations impose trade barriers on emerging economies to protect their domestic industries, impacting global income distribution.