It's important to know that "n" means how many sets or copies of chromosomes rather than how many chromosomes in general. If the latter were true, then "2n" would mean a cell has twice as many chromosomes as a cell that is "n." In reality, all cells have all chromosomes, including gamates.

So an organism usually has "2n" cells because its parents donated one copy each to make the organism. But the organisms' gamates are "n" cells so that when they fuse to create a new organism it will be "2n" as well. 

You can also remember this by knowing that mitosis goes from 2n to 2n and that meiosis goes from 2n to 2n to n.

The correct answer is metaphase because this is the stage of mitosis where the chromosomes come together and line up in the middle forming the metaphase plate. These chromosomes then get pulled to either side in the anaphase stage.

The order of mitosis is the following:

1. Prophase
2. Prometaphase
3. Metaphase
4. Anaphase
5. Telophase

Recombination or crossing over happens primarily in Prophase I. In anaphase and telophase, the DNA strands are separated and cannot recombine. In metaphase, homologous chromosomes line up with each other, but do not recombine.

Mitosis and meiosis share many procedural similarities, however, it is important to remember that mitosis makes identical cells while meiosis allows genetic variability between cells.

Meiosis takes place in sex cells, which are capable of creating millions of different variations of offspring. Mitosis takes place in normal cells involved in growth and regeneration within a single, uniform organism.

Meiosis includes two divisions, resulting in two pairs of haploid cells, while mitosis only involves a single division.

Both meiotic and mitotic divisions share the same phases, though there are some differences in the activity of these phases. Mutation can, and does, occur in both mitosis and meiosis. Crossing over, however, is unique to meiosis.

Crossing over ensures genetic variability as it results in daughter cells with different genetic material than their parent cells. This occurs during meiosis I, but is not seen in mitosis or meiosis II.

Both mitosis and meiosis occur in humans. Somatic cells (body cells) divide via mitosis, while gametes (sex cells) divide via meiosis. Because of actions such as crossing over, meiosis results in a higher genetic variability than mitosis.

Prokaryotes, such as bacteria, reproduce asexually, and are incapable of meiosis.

Turner syndrome occurs when a person is missing one sex chromosome, and only has one X-chromosome. As a result, they will be female, and may suffer a variety of symptoms. This is an example of monosomy, in which a person only has one chromosome, when they should have two.

The other three choices are examples of trisomy. Klinefelter is an instance of sex-linked trisomy, with a karyotype of XXY. Down syndrome is cause by trisomy 21, and Edwards syndrome is caused by trisomy 18.

Non-disjunction occurs when sister chromatids fail to separate during meiosis, leading subsequent daughter cells to have an unequal number of chromosomes. This can result in a cell having one extra chromosome (trisomy), or missing one chromosome (monosomy).

The following shows non-disjuction occuring in metaphase II of meiosis. "X" represents a chromosome, "|" represents a cell membrane, and "\" and "/" represent chromatids. When a sperm cell fuses with the egg formed to the left of the membrane (|), it will result in trisomy. When a sperm fuses with the egg to the right of the membrane (|), it will result in monosomy.

Metaphase I: X  X

Telophase I: X | X

Metaphase II: / \

Telophase II: / \ |