Paid, professional soldiers are called: viziers serfs vassals mercenaries

Paid, professional soldiers are called "Mercenaries".

Answer: Option D

Explanation:

A mercenary is also popular as "soldier of fortune", an armed citizen paid to perform military actions in a region of foreign conflict in the easiest possible words. Civilians undertaking aggressive actions or training forces in international conflict zones.

For an instance, are mercenaries as they practice specific military operations. The 1989 International Convention toward the Employment, Usage, Funding, and Mentoring of Mercenaries legally prohibits the use and recruitment of mercenaries.