Answer:
The correct combination of the sentences is Some dogs play fetch but, others would rather nap.
Step-by-step explanation:
From the question above asking the best combination of the sentences, using the conjunction "but" is the best way to link the two clauses.
The first sentence, "Some dogs play fetch" is an independent clause.
The second sentence "Others would rather nap" is a dependent/subordinate clause.
The best way to join an independent and subordinate clause is by using the conjunction "but".