AES difference propagation when does active S box count reduce in AES difference propagation?
what is the relationship between active S-boxes and MixColumns?
can you please explain it with an example?
The answer to the second part of your question:

The AES MixColumns operator ensures that the 8 bytes (4 in the input column 4 in the output column) form the codewords of an MDS code over $GF(2^8)$, which means the minimum weight of the code, which is 5, equals the number of nonzero bytes.

Any nonzer byte contributes 1 to the minimum weight, by definition of Hamming Weight over $GF(2^8)$. A nonzero symbol has weight 1, regardless of how many bits of the eight is nonzero.

So the number of active Sboxes is at least five.

See the answer to this question 
and the following link for more details: