The most appropriate answer to why some people in the Soviet Union, Cuba, and East Germany participated in the black market economy is:
A) they wanted to make a little extra money and gain access to goods that were hard to find in their countries.
In these countries, centrally planned economies often led to shortages of various goods, and the black market provided an avenue for people to acquire items that were otherwise unavailable or scarce in official retail channels. Additionally, participating in the black market allowed individuals to supplement their income and improve their standard of living in a context where official wages and supply were often insufficient.