The time when Europe had the capability to go off on adventures of its own or be a threat to the US is past. European economies are no longer big enough proportionately. Compare a lit of the world's largest economies from 40 years ago to now.
Now compare that to the same numbers from 40 years ago.
Then do the same with nominal GDP which is a better measure for this IMO as you cannot buy anything in global markets at PPP.
I would also say (d) make it clear to the Soviet Union that, if they invaded West Germany, it would not just be an intra-European war; it would be a world war.
Sure, but that rationale went away in the early 1990s and they stayed for thirty years longer than that.
True. Institutions like that have a lot of momentum, and they sometimes outlive their original rationale.
But on the other hand, Russian aggression is back in the headlines these days...
Aircraft carriers cannot replace military bases. That is why they stayed.
The US can support all of its military operations in Africa and the Middle East from its vast cold war era infrastructure in Europe.
"huge number of troops in Germany in the first place?"
I wouldn't call 35000 troops of mostly non-combat specializations a "huge number". In the Cold War, the American presence in West Germany was an order of magnitude larger, about quarter a million soldiers.
"Defence, or occupation?"
Neither/nor. Logistics for various distant operations. Rhineland is a very practical stop located between the US and the Middle East + North Africa (MENA), closer to MENA, with highly developed maintenance and repair facilities, communication facilities, top-notch hospitals for casualties etc.
If a GI Joe gets seriously injured in Kabul or Baghdad or now possibly Tehran, they are airlifted to Ramstein. If an armored vehicle gets damaged by a roadside bomb and the local mechanics cannot fix it on the spot, dtto.
General Hodges wrote several articles on how dumb the current administration is to alienate Europe, because without access to such facilities, any deployment in the wider region becomes very challenging. Even the original invasion of Iraq was somewhat complicated by the Irish refusing the use of Shannon airport for US military aircraft.
An intricate plan designed to further US interests, wrecked by an ignoramus who didn't understand it.