Final answer:
The combining form that means straight is 'ortho-'. It is used in terms like orthodontics and orthopedics, which relate to correcting or straightening teeth and bones or muscles.
Step-by-step explanation:
Out of the options listed, the combining form that means straight is ortho-. This prefix is of Greek origin and is commonly used in medical terminology. For example, orthodontics is a branch of dentistry that deals with the correction of straightening teeth. It is also seen in the word orthopedics, which pertains to the branch of medicine dealing with the correction or prevention of deformities of the bones or muscles. The other options, trans, dia, supra, and esto, do not mean straight. Instead, they translate to 'across', 'through', 'above', and is not a recognized combining form in medical terms, respectively.