Step 1: Analyze Assertion (A): All noble gases are monatomic.
Noble gases (He, Ne, Ar, Kr, Xe, Rn) have completely filled valence shells: He has $1s^2$ and the rest have $ns^2np^6$. Because their valence shells are full, noble gases have no tendency to form bonds and exist as isolated atoms. Assertion (A) is correct.
Step 2: Identify the true reason for monatomic nature.
The monatomic nature arises from the stable, completely filled electron configuration. High ionization energy and negligible electron affinity mean there is no thermodynamic driving force for bond formation.
Step 3: Analyze Reason (R): Noble gases have very low melting and boiling points.
Noble gas atoms interact only through weak London dispersion forces, since they have no permanent dipole and no hydrogen bonding. These forces require very little energy to overcome, giving very low melting and boiling points. Reason (R) is also correct.
Step 4: Examine whether (R) explains (A).
The monatomic nature (A) is caused by the filled electron configuration. The low melting and boiling points (R) are a consequence of the atoms already being monatomic (no bonds to break), not the reason for it.
Step 5: Determine the relationship between (A) and (R).
Both (A) and (R) are true but stem from the same underlying cause (filled valence shells). The low boiling points are a result of monatomic existence, not its explanation.
Step 6: State the final answer.
\[ \boxed{\text{Both (A) and (R) are correct but (R) is not the correct explanation of (A).}} \]