Which is the direct object of the sentence?

The events that took place or for short "the events" is the direct object of your sentence. Correct answer is A

a). Galileo did not invent the telescope, but on hearing, in 1609, that such an optical instrument had been made, he

Explanation:

The first sentence i.e. option a rectifies the given sentence correctly. It correctly employs past perfect tense where the first action(telescope had been made) is written in 'had + V3' and the latter action(built his own version) in 'V2.' The use of 'but' helps in displaying the contrast effectively.

The second option is incorrect as the general facts are stated in the past indefinite and it also contains punctuation error, misses the subject 'he' for the verb 'built.' The third option also contains the verb error of inappropriate use of past perfect and the error in the use of the pronoun 'he' succeeding 'even though' as the subject is given at the start of the sentence. The options d and e are wrong due to the inappropriate use of 'even though.' Thus, option a is the correct answer.