What is the probability that, in an organism with a haploid number of 12, a sperm will be formed that contains all 12 chromosomes whose centromeres were derived from maternal homologs?

Having 2 chromosomes that are divided will give you a chance of getting one of them of 50%. Calculation of the probability as follows:

Probability = 0.5^12

Probability = 0.00024

So, you have a chance of 1 in 4167. Hope this answers the question.