#05AB1E , 5 bytes

ŒIù.M

Try it online or verify the smaller test cases or verify the larger test case (which is a bit slow).

Explanation:

Π# Push all substrings of the (implicit) input-string
 Iù # Only keep substrings of a length equal to the second input-integer
 .M # Only keep the most frequent item of the remaining substrings
 # (after which it is output implicitly as result)

05AB1E , 5 bytes

ŒIù.M

Try it online or verify the smaller test cases or verify the larger test case (which is a bit slow).

Explanation:

Π# Push all substrings of the (implicit) input-string
 Iù # Only keep substrings of a length equal to the second input-integer
 .M # Only keep the most frequent item of the remaining substrings
 # (after which it is output implicitly as result)

#05AB1E, 5 bytes

ŒIù.M

Try it online.

#05AB1E, 5 bytes

ŒIù.M

Try it online or verify the smaller test cases or verify the larger test case (which is a bit slow).

Explanation:

Π# Push all substrings of the (implicit) input-string
 Iù # Only keep substrings of a length equal to the second input-integer
 .M # Only keep the most frequent item of the remaining substrings
 # (after which it is output implicitly as result)