Step 1: Understanding the rule of articles.
Definite articles like “the” are used before the names of mountain ranges, rivers, seas, and groups of islands to specify them uniquely.
Step 2: Apply the rule.
“The Alps” refers to a specific and well-known mountain range in Europe, hence it must take the definite article “the.”
Step 3: Analyze the options.
- (a) A: Used with singular countable nouns, not suitable for plural proper nouns like “Alps.”
- (b) An: Used before vowel sounds, not applicable here.
- (c) The: Correct; definite article used for famous ranges or landmarks.
- (d) No article: Incorrect; article omission would make the sentence incomplete.
Step 4: Constructing the correct sentence.
“The Alps are the longest mountain range in Europe.”
Step 5: Conclusion.
Thus, the correct article is “The.”