How many nonisomorphic fields of order < 50 are there?
10
15
22
49
Wrong. There is one for every prime power. There are even more than 10 prime numbers less than 50.
Wrong. There are 15 primes less than 50, viz.
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47.
But the prime powers should also be counted.
Right.
This is exactly the number of prime powers in {1, ..., 50}: there are 15 first powers of primes, 4 second powers, 2 cubes, and 1 fourth power.
No. There is one for every prime power, and there are not this many prime powers less than 50.