Answer:
The correct option is number 1. Hydroxyapatite contributes most directly to the hardness of bones
Step-by-step explanation:
Hydroxyapatite can be described as a naturally occurring biological molecule of high importance. It provides hardness the bones and teeth. Hence, option 1 is correct.
Periosteum can be defined as the outer layer of a bone which functions as an attachment site for the skeletal muscles. Osteoid is a structure that is made before a bone occurs in its mature form. Hence all the other options 2, 3 and 4 are false.