The correct option is C the
In the first sentence, 'a' is used before 'telescope' because it's mentioned for the first time. In the last sentence, the correct answer should be 'the' because by then both the speaker and the listener know the identity of this noun. Hence Option C is the correct answer. Option A is used before a noun which begins with a vowel. Since the noun over her does not begin with a vowel, it isn't the correct option. Option B is used when the identity of the noun is not known to the listener or even the speaker in some cases, like in the first sentence 'John had bought a telescope'. Hence Option B is incorrect. An article is not required when the noun is a proper noun in most cases, or if the noun is a general non-count noun. Since the telescope is a singular definite noun, Option D is incorrect as well.