An example of a rational decision would be:

Choosing to study for an exam instead of going out with friends illustrates a rational decision because it reflects a consideration of long-term benefits over short-term gratification. In this scenario, the individual recognizes that studying is likely to lead to better academic performance, which can have positive implications for their future, such as achieving higher grades, improving knowledge, and potentially opening up more opportunities for success. This decision demonstrates an evaluation of trade-offs, where the immediate pleasure of socializing is weighed against the future gains of educational achievement, showing an understanding of the consequences of the choices being made.

In contrast, the other options involve decisions that do not necessarily align with rational decision-making based on logical assessment of benefits versus costs. For example, choosing to buy a product solely based on brand preference lacks a comprehensive evaluation of value compared to alternatives. Believing one will get promoted based solely on hard work overlooks the necessity of other factors like company dynamics and opportunities that might not be in the individual's control. Lastly, spending all savings on an entertainment system demonstrates a focus on a single, immediate desire without proper consideration for future financial security, which contradicts the principles of rational decision-making in economics.