Step 1: Frame the question.
Among Helium, Neon, Argon, and Xenon we want the noble gas that is most reactive and forms the largest set of compounds.
Step 2: Recall why noble gases are inert.
Their valence shells are completely filled ($ns^2np^6$, or $1s^2$ for He), so they have little urge to gain, lose, or share electrons.
Step 3: Track the trend down the group.
Going down from Helium toward Radon, atomic size grows and ionization enthalpy falls. The outer electrons get held more loosely, so reactivity rises down the group.
Step 4: Compare the four options.
Of Helium, Neon, Argon, and Xenon, Xenon sits lowest, so it has the biggest atom and the smallest ionization enthalpy. Helium, Neon, and Argon are essentially unreactive.
Step 5: Back it up with chemistry.
Xenon's ease of losing electrons let Neil Bartlett make the first noble gas compound $\text{XePtF}_6$, and Xenon readily forms $\text{XeF}_2$, $\text{XeF}_4$, $\text{XeF}_6$, $\text{XeO}_3$, and more with fluorine and oxygen.
Step 6: Conclude.
Xenon is the most reactive and forms the most compounds, which is option (D).
\[ \boxed{\text{Xenon}} \]