The correct answer is C. When there are multiple options and factors to consider.
Decision-making becomes more complex in situations where there are many options to choose from and various factors that influence the decision. This complexity arises because individuals or groups must evaluate the potential outcomes, weigh the pros and cons of each option, consider the relevance of different factors, and deal with uncertainties or lack of information.
In conclusion, having multiple options and various influencing factors is the primary reason why decision-making complexity increases.