What is the stage of mitosis where DNA is condensing into organized chromosomes and the nuclear envelope disappears?

Correct answer is prophase. 

During prophase, the chromosomes condense, the nucleolus disappears, and the nuclear envelope breaks down. 

During prophase, the complex of DNA and proteins contained in the nucleus, known as chromatin, condenses. The chromatin coils and becomes increasingly compact, resulting in the formation of visible chromosomes. Chromosomes are made of a single piece of DNA that is highly organized. 

Prophase is the first phase of mitosis, the process that separates the duplicated genetic material carried in the nucleus of a parent cell into two identical daughter cells. During prophase, the complex of DNA and proteins contained in the nucleus, known as chromatin, condenses. The chromatin coils and becomes increasingly compact, resulting in the formation of visible chromosomes. Chromosomes are made of a single piece of DNA that is highly organized. The replicated chromosomes have an X shape and are called sister chromatids. The sister chromatids are pairs of identical copies of DNA joined at a point called the centromere. Then, a structure called the mitotic spindle begins to form. The mitotic spindle is made of long proteins called microtubules that begin forming at opposite ends of the cell. The spindle will be responsible for separating the sister chromatids into two cells.