This first historian was the author of the "History of the Peloponnesian War":

a. Herodotus
c. Thucydides
b. Plato
d. Socrates

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The first step to finding the answer to your question is to understand what is being asked. The question is asking about the author of the "History of the Peloponnesian War." To find the answer, we need to identify the author of this historical work.

The next step is to examine the options provided: Herodotus, Thucydides, Plato, and Socrates. To determine the correct answer, we can use our knowledge of ancient Greek historians. By process of elimination, we can rule out option b (Plato) and option d (Socrates), as they were both philosophers and not historians.

This leaves us with two options: Herodotus and Thucydides. Both were renowned ancient Greek historians, but Thucydides is the correct answer. He is widely regarded as the author of the "History of the Peloponnesian War," which is considered one of the most influential historical works of the ancient world.

Therefore, the correct answer is:

c. Thucydides