Here, ‘Guitar’ is a common noun and we can easily figure out that the speaker’s father will buy her/him single guitar. Hence, indefinite article ‘a’ will be used as word starts with a consonant sound.

The identity of the apartment is known to the reader because ‘apartment’ is mentioned in the first sentence. Hence, definite article ‘the’ will be used.

‘A’ is used before unspecific, singular nouns that begin with a consonant sound. The sentence refers to an unspecific flat. ‘The’ is used before specific or unique places/things. Here, ‘city centre’ is the name of a place which is unique (only one).

In the sentence ‘new car’ is general noun. The word (new) has consonant sound and therefore ‘a’ article is used.

In the first blank, article “the” will be used because article ‘the’ is used to refer to something definite. In this sentence, speaker is referring to some specified number of pupils.
In the second blank, “no article or zero article” will be used because ‘no article’ is used with ‘school, colleges’ (if name of the school is not mentioned or there is no use of ‘of’).

The first blank won’t take any article as ‘solar energy’ is an abstract noun. The second and third blank shall take ‘the’ as the sentence talks about a particular location on a particular star.

‘The’ article will be used because a specific train ‘Pandiyan express’ is talked about.

In the first blank, article “the” will be used because the statement is mentioning a ‘specific day’.
Similarly, in the second blank, article “the” will be used because the statement is mentioning a ‘specific race’.