The correct answer is: A. Tooth caries.
Fluoride is a mineral that helps to prevent tooth decay. It does this by making the teeth more resistant to acid attacks from plaque bacteria. When fluoride is deficient, the teeth are more likely to develop tooth decay.
Mottled teeth are a cosmetic problem that can be caused by excessive fluoride exposure. They are characterized by white or brown spots on the teeth.
Bending of bones is a problem that can be caused by a number of factors, including osteoporosis, rickets, and osteomalacia. These conditions are all caused by a lack of calcium or vitamin D, which are necessary for strong bones.
Stiffening of joints is a problem that can be caused by a number of factors, including arthritis, rheumatoid arthritis, and osteoarthritis. These conditions are all caused by inflammation of the joints, which can lead to pain, stiffness, and swelling.
In conclusion, the correct answer is: A. Tooth caries.