Meiosis is the process by which the reproductive cells are created. Those reproductive cells are called "gamete cells", in opposition to the other cells that are called "autosome cells". As it's goal is to promote reproduction, the gamete cells always have half of the number of chromosomes that would be found in autosome cells. This is important because two gamete cells must merge in order to ensure that reproduction takes place, and two gamete cells must become one autosome cell. So, gamete cells have an haploid number of chromosomes (n) and the autosome cells have a diploid number of chromosomes (2n, meaning the double of the amount of chromosomes in n).
Therefore, the correct answer is:
b) gamete cells and d) haploid number of chromosomes