The correct answer to this open question is the following.
Although there are no options attached, we can say that the major factor in spurring on Iran's development of nuclear weapons in the 1980s was the victory of the Revolutionary forces in Iran, that overthrew the Shah, the last one that ruled over Iran. So in 1981, after the civil war ended, Iran authorities decided to continue the development of nuclear weapons as a way to defend the sovereignty of the country. In that time, the Soviet Union decided to support Iran sending expert scientists to help Iran to develop its own research program.