Poor flexibility may be caused by a. muscle tightness b. muscle looseness c. youth d. high levels of fitness

(a) muscles tightness.

Explanation:

Inappropriate flexibility will have adverse effect on the body in three ways:

The requirement of joint movement through a motion of full range to maintain the cartilage's health and other structures in the joint with  blood supply increase and  joint structures nutrients with an increase of synovial joint fluid.

Inflexible muscles  tire more quickly, causing  muscle  groups which are opposing to work harder. Muscle fatigue can results in muscular injuries and the non ability of the muscles to protect joints from high severe injuries.

Decreased flexibility may also results in structures abnormal stress and distance of tissues from the initial site of non flexibility. For example, tendonitis in the knee can be related to  tightness of calf.