Fill in the blank with the appropriate article:
Copper is________ useful metal.
a) An
b) The
c) No article
d) A

Hint: The words "a," "an," and "the” are the articles. They show whether something is specific or unspecific.

Complete Solution:
There are two different types of article, the definite article and the indefinite article.
Definite Article - The word ‘The’ is used to portray that the speaker is referring to something specific and not talking in a general sense.
Indefinite Article - The indefinite articles are ‘a’ and ‘an’.
If a noun begins with a vowel, ‘an’ is used.
If a noun begins with a consonant, ‘a’ is used.
Exception: The choice between the two indefinite articles – a & an – is determined by sound. Words beginning with consonant sounds use ‘a’ and words beginning with vowel sounds use ‘an’. Example - a university, a union, a useful book.
Option a ‘An’ is incorrect because ‘useful’ has a consonant sound.
Option b ‘The’ is also incorrect because the sentence doesn’t specify any particular object but is talking about an object in general.
Option d is correct because it conforms with the rules of the articles. It falls in the category of exception which specifies that ‘a’ is used before a vowel that has consonant sound.

Note: The rules must be understood properly along with its exception and the reason behind such an exception.