Answer:

The correct answer is option 'B' - an.

When using articles before singular countable nouns, there are two main options: 'a' and 'an'. The choice between 'a' and 'an' depends on the sound that follows the article, not the actual letter.

If the noun begins with a consonant sound, we use 'a'. If the noun begins with a vowel sound, we use 'an'.

Some examples of nouns beginning with a vowel sound are:

- an apple
- an elephant
- an orange

Explanation:

The reason why we use 'an' before a singular countable noun beginning with a vowel sound is for easier pronunciation. The use of 'an' instead of 'a' helps to avoid the awkwardness of having two vowel sounds in a row.

For example, it would be awkward to say "a apple" because the 'a' sound clashes with the beginning vowel sound of 'apple'. However, it sounds much smoother to say "an apple" because the 'n' sound separates the two vowel sounds.

In terms of grammar, it is important to note that the choice between 'a' and 'an' is based on the sound of the word, not the actual letter. For example, we say "an hour" because the 'h' in 'hour' is silent, so the word begins with a vowel sound.

It is also worth mentioning that the choice between 'a' and 'an' does not depend on the actual vowel or consonant letter, but rather on the sound that follows it. For example, we say "a university" because the 'u' in 'university' is pronounced as a consonant sound.

Examples:

- an apple (pronounced as "an apple")
- an elephant (pronounced as "an elephant")
- an orange (pronounced as "an orange")

In conclusion, the correct article to use before a singular countable noun beginning with a vowel sound is 'an'. This choice helps to ensure smooth and clear pronunciation.