When people believe in the existence of one god, they are practicing
a.
atheism
b.
paganism
c.
polytheism
d.
monotheism

The correct answer is D. Monotheism

Explanation

Monotheism is a religious current that is based on the existence of only one god or deidity, nevertheless that is not the only religious current. Indeed, through history there have been a lot of religions for example in the earliest communities, the religion was based in natural phenomena and each one was related with one God (polytheism). Later, when  Christianism, Catholicism, Islam, and Judaism originated, religions were mainly focused on one God (monotheism) that was the most important and unique deidity in these religions. So, the correct answer is D. Monotheism.

nothing certain is known about what happened, but it seems that the country declined gradually, perhaps due to bad government, or, more likely, due to a series of bad harvests, which resulted in widespread and recurrent famine.

2018 means 2018 ce or common era

there have been other eras too including bce, which included way more years.