What type of waveform is represented by a straight red line in an illustration?

A straight red line in an illustration typically represents direct current (DC). This is because direct current is characterized by a constant flow of electric charge in a single direction, and visually, it appears as a straight line when graphed over time.

In contrast, alternating current (AC) involves a periodic change in direction of the electric charge, leading to a waveform that fluctuates above and below a central line, often depicted as a sine wave. Unrectified alternating current would still show these fluctuations without any modification, while rectified alternating current refers to AC that has been converted to DC through a process that removes the negative portions of the waveform, resulting in a waveform that does not drop below zero but may still have variations.

Therefore, the straight line indicating a consistent flow confirms the nature of direct current, making it the correct answer.