The correct option is A the
Option A: 'The' is a definite article that modifies nouns that are
specific.
It's also used when we refer to a particular century. '1800's' refers to the 19th century. Hence 'the' is the most
suitable article.
So, option A is correct.
Options B and C: 'A' and 'an' are both indefinite articles. They refer
to nouns that are not specific. This means that we do not know the
identity of the noun. For example, 'a book', 'an elephant', etc.
(The
only difference between 'a' and 'an' is that 'a' is used for nouns that
begin with a consonant, like 'b' in book', and 'an' is used for nouns
beginning with a vowel, like 'e' in 'elephant'.)
The noun here '1800's' refers to a particular century. Hence B and C are incorrect.
Option
D: An article is not required when the noun is spoken of in general
sense, and when the noun is uncountable. The word here is neither, hence
D is incorrect.