Answer:
b. sequences containing repeats longer than the DNA fragments to assemble
Step-by-step explanation:
If the DNA fragments is full of repeats, it's difficult to assemble the genome since there's no way to connect the ends of the read or fragment to another one.
In the opposite way, if the DNA fragment contain repeats shorter, then the ends of the fragment will be a sequence which is not part of the repeat and ir will be easy to bind the end to another fragment. This is necessary to form the contigs which are overlapping DNA fragments.