Which of the following actions is illegal while operating a vehicle?

Answer: A — Driving without a valid permit.

Driving without a valid permit is illegal because it violates the licensing laws established to ensure that all drivers have demonstrated the necessary knowledge and skills to safely operate a vehicle. Obtaining a valid permit confirms that the individual has completed the required training and understands the rules of the road, ensuring safety for both the driver and others on the road. Without this permit, the driver may lack the necessary experience and understanding, increasing the risk of accidents. In contrast, driving during daylight hours is permissible and does not pose any legal issues. Likewise, carrying a passenger under 18 is generally allowed; this action may be subject to specific restrictions based on state law, but it is not inherently illegal. Finally, using turn signals is not only legal but is also a crucial safety practice that helps communicate intentions to other road users.