$(i)$ $(a)$ What do you understand by $1 \text{ amu}$ or $1 \text{ u}$?
$(b)$ How is it linked with relative atomic mass?
$(ii)$ How do we know the presence of atoms if they do not exist independently for most of the elements?

(N/A) $(i)$ $(a)$ One atomic mass unit ($1 \text{ amu}$ or $1 \text{ u}$) is a mass unit equal to exactly $\frac{1}{12}$th the mass of one $C-12$ atom.
$(b)$ The relative atomic mass of an element is defined as the average mass of one atom of the element compared to $\frac{1}{12}$th the mass of one $C-12$ atom.
$(ii)$ Atoms of most elements are highly reactive and cannot exist independently. However,they combine to form molecules and ions. These particles aggregate in large numbers to form matter,which we can see,feel,or touch,thereby confirming the presence of atoms.