The correct option is A Both assertion & reason are true & the reason is a correct explanation of the assertion.
Assertion: M-phase is a dramatic phase as there are major changes and events taking place during this phase. The M-phase is divided into karyokinesis which have the events: prophase, metaphase, anaphase and telophase. Finally resulting in the formation of 2 daughter cells after cytokinesis.
Reason: The mitotic events of anaphase and telophase result in the reorganisation of components of the cell like the chromosomes, cell organelles in the cytoplasm which are divided equally during the formation of daughter cells. This makes the M-phase a dramatic event.
Hence both assertion and reason are true and reason is the correct explanation of assertion.