Step 1: Apply the First Law of Thermodynamics for an Ideal Gas
For an ideal gas:
\[
\Delta H = \Delta U + R \Delta T
\]
Rearrange to solve for \( \Delta U \):
\[
\Delta U = \Delta H - R \Delta T
\]
Thus, the correct answer is \( \Delta U = \Delta H - R \Delta T \).