Which factor directly affects the income in relation to days earned?

The factor that directly affects income in relation to days earned is increasing the number of times a vehicle can be rented in one day. This is because the income generated by a rental business is fundamentally tied to how many rentals can be completed within a given time period. By optimizing the rental process to allow for more transactions in a single day, a rental company can significantly enhance its revenue potential. This strategy increases the utilization of each vehicle, leading to a higher overall income from that asset.

In contrast, while fleet maintenance costs can impact profitability, they do not directly influence the income related to rental days earned. Similarly, having a number of repeat customers enhances customer loyalty and can lead to increased revenue over time, but it does not directly correlate to the number of rental days earned in a specific timeframe. Finally, seasonal demand fluctuations can create peaks and troughs in business but do not inherently increase the frequency of rentals per day unless managed effectively.