.......... Alps are the longest mountain range in Europe.

Rule for articles with geographical features:

• We use 'the' before the names of:

  • Mountain ranges (e.g., the Himalayas, the Andes, the Alps)

  • Rivers, seas, oceans, deserts, and groups of islands

Since Alps is the name of a mountain range, it must take the definite article 'the'.

Thus, the correct sentence is: The Alps are the longest mountain range in Europe.

The other options are incorrect because:

• A. A - 'A' is used before singular, countable nouns. 'Alps' is plural and a proper geographical name.

• B. An - Used only before singular nouns starting with a vowel sound.

• D. No article - Some proper nouns do not take articles (e.g., Mount Everest, Europe), but mountain ranges always require 'the'.