Final answer:
The best-case time complexity for inserting a key into a hash table that is storing N keys is O(1), which represents constant time complexity irrespective of the number of elements in the hash table.
Step-by-step explanation:
The question concerns the time complexity for inserting a key into a hash table that already contains N keys. When a hash table is properly designed with a good hash function and is not overloaded, the insertion process can be very efficient. The best-case scenario for hash table insertion, considering that there are no collisions or that a collision resolution scheme like chaining or open addressing is effectively handled, is O(1). This means that the insertion operation can be done in constant time, regardless of the number of elements in the table. Hence, the correct option is (c).O(1).