Answer:
A major change over long periods of time is best described as macroevolution.
Step-by-step explanation:
Macroevolution would be referring to evolution on a larger scale where populations speciate etc. A, B and C are various forms of speciation and microevolution would be referring to smaller changes in evolutionary patterns e.g. in adaptive or physical traits which may not be deemed 'significant' enough to be considered a speciation event.