while I know the answer to the question, I won’t tell you. It appears to me that this is a question on a test for which you are to find an answer based on the study material you have.
Taking the effort is takes to study is an important part of the process of learning. It will teach you the ‘why’ of an answer.
Once you are out in the fitness world for a while, you will find that the sit and reach test is out of date. I recommmend that you look at ROM and movement in general as a joint specific and multi joint related aspect of fitness/health. And dynamic movement is far more important than simple ROM at any joint. How does the client move and what is happening at the feet, ankles, knees, hips, low back, thoracic spine, and shoulders during movement. I know you need to pass a test to get going. After that really get going and open your mind to the amazing ability of the human body.