Answer: I thought about napkins for a long time, but I could not come up with something magnificent.
Explanation:
1. I thought about napkins for a long time I could not come up with something magnificent ----- Here there is no conjunction to enable the two sentence make a contrasting or co-joining meaning
2. I thought about napkins for a long time, and I could not come up with something magnificent.
The conjunction AND is not the best way to combine the two sentences. "and" in sentences is used to connect two actions, with one following the other.
3. I thought about napkins for a long time, so I could not come up with something magnificent
So is used as a subordinating conjunction used to express reason for an action and therefore cannot be applied here.
4. I thought about napkins for a long time, but I could not come up with something magnificent.
The best way to combine the sentences is the use of but which is used here to express a contrast( difference ) in an action
