The Persian king who undertook an invasion of the greek mainland in the early fifth century was Darius. Thus the correct option is D.
The term invasion is refer to the acquisition of land. When in the situation of war one country uses its power to seek control over another with the use of military forces the process is called an invasion.
Early in the fifth century, the Persian king Darius led an invasion of the Greek mainland by spending four years putting down the rebellion. He becomes the king of Persia at the age of 28.
Therefore, option D is appropriate.
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