the answer is: b. unintended consequences of a change that are not immediately identifiable but are felt only with time.
For example, let's say that the government decided to invest in conserving the natural environment in order to maintain food and water reserves. (primary effect)
But, after the natural environment is conserved, the country experience an unpredictable increase in tourism revenue because the beauty of the nature attract a lot of people from other countries. (this is what is called as secondary effect).