How can poor posture result in back pain? A.
Bad posture can result in tight muscles.
B.
Bad posture hyperextends the spinal column.
C.
Bad posture increases back flexibility.
D.
Bad posture reduces stress on joints and tendons

If the person is young, bad posture can result in tight muscles which can cause pain within hours and OVER TIME (like months or YEARS) it could cause your spinal column to hyperextend and cause disk herniations which can be painful. So if you can only pick one answer, I would pick A, Poor posture can result in tight muscles. Think about when you get a sore neck from texting for too long. It's from straining your neck muscles to support the weight of you head in an unnatural (hunched over) position, AKA poor posture. But if you can pick more than one answer than I would go for A and B, because older people develop chronic pain and spinal arthritis (osteoarthritis) from everyday wear and tear on the spine from poor posture.

Explanation: Some tests let you choose more than one answer, I'm giving you the first answer to go for (Answer A) and a second one, (Answer B) if the test is saying "choose all correct answers."