Answer:
a. Galileo did not invent the telescope, but on hearing, in 1609, that such an optical instrument had been made, he
Step-by-step explanation:
Looking at the given sentences, the first thing we need to ascertain is the tense form. The use of "did not" makes it a past tense which is also supported by the use of the past perfect tense "had been". Moreover, the conjunction "but" perfectly compares/ presents a contrast between two ideas or opinions. Thus, option A is correct.
The other options such as B, C, D, and E are all incorrect.
Option B is wrong as it uses "had not invented" which should be in the simple past tense form. Moreover, it did not specify the pronoun "he" for the device built in the latter part of the sentence.
Option C is wrong as it follows the same verb tense form error as in option B.
Option D is wrong with the absence of the pronoun "he" as in option B. Also, the use of "even though" presents faulty sentence structuring.
Option E is wrong as in option D and also the redundant use of the "but" after the use of another comparing word "even though".