Final answer:
Condensin is the protein that compacts chromosomes during the early prophase of mitosis, preparing them for proper segregation.
Step-by-step explanation:
The protein responsible for compacting chromosomes in the early prophase of mitosis is condensin. These proteins help sister chromatids coil during prophase, leading to the condensation and coiling of DNA into chromosomes which is essential for their proper segregation during mitosis. Cohesins are also important as they hold the sister chromatids together, but it is condensin that is primarily responsible for the compaction process.