The correct answer is b) arthr o.
Explanation: The combining form ‘arthr o’ comes from the Greek word ‘arthron’, which means joint. It is commonly used in medical terminology to refer to anything related to joints, such as arthritis (inflammation of the joints). On the other hand, ‘ankyl o’ relates to stiffness or fusion of joints, ‘orth o’ pertains to straight or normal, and ‘burs o’ refers to a bursa, which is a fluid-filled sac that reduces friction between tissues. Thus, when discussing terms that relate to joints specifically, ‘arthr o’ is the most appropriate option.