According to the ISO 9001:2015 document, the organisation must continually improve the suitability, adequacy, and effectiveness of the quality management system1. However, among the six options given, only effectiveness is directly mentioned as an aspect of the quality management system that must be continually improved. Therefore, C is one of the correct answers.

Efficiency, on the other hand, is not explicitly stated as an aspect of the quality management system that must be continually improved, but it is implied by the quality management principle of improvement, which states that successful organisations have an ongoing focus on improvement2. One of the key benefits of applying this principle is improving operational effectiveness and efficiency2. Therefore, E is another correct answer.

Suitability, adaptability, responsiveness, and applicability are not aspects of the quality management system that must be continually improved, according to the ISO 9001:2015 document. They may be related to the quality management system, but they are not the focus of continual improvement.

Therefore, the correct answer is C and E.