A closed-loop system is a control system that uses feedback to maintain a desired output. It continuously monitors its output, compares it to a target value (setpoint), and adjusts its input to correct any deviation. The cruise control example perfectly embodies this principle. The system measures the car's actual speed (output), compares it to the set speed, and automatically adjusts the throttle (input) to maintain that speed. The description explicitly mentions holding the speed "despite outside disturbances" (like hills), which is the primary function of a feedback loop—disturbance rejection and regulation.

A. This describes an open-loop test. Monitoring the flashing frequency (output) does not feed back to control the activation of the lights (input).

B. This is an open-loop test. Reading the error memory (output) is a verification step that does not influence the motor temperature (input).

C. While an automatic wiper system is a closed-loop system, this description is less complete than D, as it focuses on activation rather than the continuous regulation and feedback process.

1. ISTQB® Certified Tester Foundation Level Automotive Software Tester Syllabus (v1.0), Section 3.2.2, "Test Environment for Hardware-in-the-Loop (HiL) Testing". This section defines a closed-loop system as one where "the outputs of the SUT are fed back to its inputs. This is necessary when the behavior of the SUT depends on the reaction of its environment". The cruise control system (D) perfectly fits this, as its behavior (throttle adjustment) depends on the vehicle's speed, which is the reaction of the environment to engine output and external forces.

2. Åström, K. J., & Murray, R. M. (2010). Feedback Systems: An Introduction for Scientists and Engineers. Princeton University Press. In Section 1.1, "What is Feedback?", cruise control is presented as the canonical example of a feedback system. The text explains how it measures speed and adjusts the throttle to "reject disturbances, such as changes in the slope of the road" (p. 1-2). This directly aligns with the description in option D.

3. Nise, N. S. (2015). Control Systems Engineering (7th ed.). Wiley. In Chapter 1, Section 1.4, "Examples of Control Systems", the automotive cruise control system is used as a primary example to illustrate the concept of a closed-loop feedback system, detailing the process of sensing speed, calculating error, and actuating the throttle to maintain the desired speed.