Answer:
maternal mtDNA
Step-by-step explanation:
While nothing but the haploid nucleus remains of the sperm during fertilisation, the ovum still has its mitochondria and other organelles. Mitochondria are capable of division and will divide according to cell's needs.
So it is the maternal mtDNa that gets passed on. Spontaneous mutations can happen at any point ofcourse and get passed on in the same way.