Which biological molecule in living organisms contains $Mg$ as an essential component?

Which of the following represents a nucleoside?

Oxidation of glucose is one of the most important reactions in a living cell. What is the number of $ATP$ molecules generated in cells from one molecule of glucose?

Which of the following contain $\alpha$-$D$-glucose units?
$a$) Cane sugar
$b$) Milk sugar
$c$) Cellulose
$d$) Amylose

Which of the following is the most important biological molecule containing $Mg$ found in living organisms?

Assertion : $Mg$ is not present in the enamel of human teeth.
Reason : $Mg$ is an essential element for the biological functions of humans.