The word that completes the sentence and means "someone who straightens teeth" is C. orthodontist.
Explanation:
1. An orthodontist is a dental specialist who focuses on correcting teeth and jaw alignment issues, such as overcrowding, overbite, underbite, and misaligned teeth.
2. Orthopedist (option A) is a doctor who specializes in the treatment of musculoskeletal disorders, such as bones, joints, ligaments, tendons, and muscles, not related to dental issues.
3. Orthography (option B) refers to the conventional spelling system of a language, which is unrelated to the context of the sentence about Lara's teeth.