The correct option is
A the/an/the
OPTION "A" is the correct answer.
EXPLANATION:
There are two types of articles:
DEFINITE ARTICLE (THE) : Article 'the' is used to refer to a DEFINITE thing or person.
INDEFINITE ARTICLE(A/AN): Article 'a/an' is used to refer to a thing or person in general. For instance: man, door etc.
Article 'a' is used with consonant sounds whereas article 'an' is used with vowel sounds.
In the first blank, article "the" will be used because article 'the' is used to refer to something specific. Here 'ground floor is a specific entity'.
In the second blank, article "an" will be used because old house is a common noun, something general. It has a vowel sound, hence, 'an' is used.
In the third blank, 'River Thames' is a proper noun, hence, article "the" will be used.