Step 1: Identify the primary stated rationale.
Russian leadership framed NATO’s eastward expansion and possible Ukrainian membership as an existential security threat, using it to justify military action.
Step 2: Separate prior events and alternative motives.
Crimea was annexed in 2014 (earlier event); resource motives (oil) or vague European ``relations'' do not match official justifications cited for 2022.
Step 3: Apply to options.
Only option (D) captures the central stated objective: preventing Ukraine’s integration with NATO.
Step 4: Conclude.
\[
\boxed{\text{Option (D): to stop Ukraine's NATO membership.}}
\]