Explanation:

Introduction:
In the given sentence, we need to identify the correct article to use before the noun "unicorn". The correct article choice depends on whether the noun is countable or uncountable, and whether it is a specific or non-specific noun.

Countable vs. Uncountable Nouns:
Countable nouns refer to things that can be counted, such as "books" or "chairs". They can be singular or plural and require an article before them. Uncountable nouns refer to things that cannot be counted, such as "water" or "advice". They do not require an article before them.

Specific vs. Non-specific Nouns:
Specific nouns refer to a particular person, place, or thing. Non-specific nouns refer to any or every member of a group or class.

Analysis of Each Option:
a) "I saw an unicorn in the forest."
- This sentence uses the indefinite article "an" before the noun "unicorn". However, "unicorn" starts with a consonant sound (pronounced "juːnɪkɔːn"), so the correct indefinite article to use would be "a". Therefore, option a) is incorrect.

b) "I saw a unicorn in the forest."
- This sentence uses the indefinite article "a" before the noun "unicorn". Since "unicorn" starts with a consonant sound, "a" is the correct choice. Therefore, option b) is correct.

c) "I saw the unicorn in the forest."
- This sentence uses the definite article "the" before the noun "unicorn". The definite article is used when referring to a specific noun that has already been mentioned or is known to both the speaker and the listener. Since there is no prior mention or specific context given in the sentence, the definite article is not appropriate. Therefore, option c) is incorrect.

Conclusion:
The correct sentence is option b) "I saw a unicorn in the forest." This sentence uses the indefinite article "a" before the noun "unicorn", which is appropriate for a non-specific, countable noun starting with a consonant sound.