Final answer:
Muscles move by contracting and pulling on bones, they work in pairs such as flexors and extensors, and they do not get smaller when exercised but can increase in size.
Step-by-step explanation:
When considering how muscles work within the body, they perform their function through a few fundamental principles. Muscles move bones by contracting, or getting shorter, and this contraction pulls on the tendons, which in turn moves the bones. Muscles work in pairs because while one muscle of the pair contracts (the flexor), the other muscle (extensor) relaxes and lengthens, and vice versa; this is necessary because muscles cannot actively extend. An example of such a muscle pair is the biceps and triceps in the upper arm that work together to bend and straighten the elbow. Given this, the correct answer to the student's question, 'Muscles: a. Work in pairs. b. Move bones by relaxing. c. Get smaller when exercised. d. All of the above.' is 'a. Work in pairs.' This is because muscles do not move bones by relaxing; instead, they move bones when they contract, and muscles do not get smaller when exercised; over time, with consistent exercise, they typically increase in size.