Before duplication 1 chromosome = 1 chromatid and after
duplication1 chromosome has 2 chromatids. if a diploid organism had
2n = 12chromosomes, how many chromosomes and chromatids would be
presentin the cell after: a) the first division of meiosis b) the
seconddivision of meiosis c) a mitotic division?

a) the first division of meiosis: 6 chromosomes, 12 chromatids in each cell.

b) the  second division of meiosis: 6 chromosomes, 6 chromatids in each cell.

c) a mitotic division: 12 chromosomes, 12 chromatids in each cell.

Explanation:

Meiosis-I includes separation of homologous chromosomes to the opposite poles during anaphase-I and thereby, reduces the chromosome number to half in each daughter cell. Therefore, after meiosis-I, each daughter cell would have 6 chromosomes and 12 chromatids.  

Meiosis-II includes splitting of centromere and separation of sister chromatids to the opposite poles during anaphase-II. Therefore, after meiosis-II, each daughter cell would have 6 chromosomes and 6 chromatids.  

Mitosis includes splitting of centromere and separation of sister chromatids to the opposite poles during anaphase. Therefore, after mitosis, each daughter cell would have 12 chromosomes and 12 chromatids.