What must happen before a cell can begin mitosis?
a mitotic spindle must form.
the nucleus must divide in two.
the sister chromatids must be separated.
the nuclear envelope must break up.
the chromosomes must be duplicated.

The correct answer is- The chromosomes must be duplicated.

Explanation:

Mitosis is a cell division in which a cell is divided into two equal daughter cells. Each cell is diploid and contain the same number of chromosomes. Cell enters into mitosis after interphase.

Interphase consists of 3 stages G1, S, and G2 phase. In G1 phase cell grows, organelle becomes doubles, synthesis of precursors for DNA replication takes place.  

In the synthesis phase DNA replication takes place and chromosome becomes double, and in G2 phase cell continues to grow and synthesis proteins needed for mitosis phase.

Therefore, chromosome must be duplicated before a cell can begins mitosis otherwise equal distribution of chromosomes will not be possible.