Answer:
The answer is letter D) continuing for a long time.
Explanation:
When we do not know the meaning of a certain word, we can try to guess it by analyzing the context in which it is inserted. In this particular case, the word "protracted" accompanies "period". The speaker had already revealed previously that Ptolemy's work influenced sixty generations, which means his work was important and influential for a very long period of time. Thus, when the speaker continues and restates that Ptolemy was a success and the "unchallenged instructor of the human race for such a protracted period," we already know that he means Ptolemy was a success for a very long time. Thus, the best assumption would be letter D) continuing for a long time.
As a matter of fact, protracted is an adjective that refers to something that lasts for a long time or, at least, longer than expected.
