Learn Spanish (Spain): ¿Cuál es la montaña más alta del mundo? - Which is the highest mountain in the world?

To say "the highest" (a relative superlative), Spanish uses the pattern: definite article (la) + noun (montaña) + más + adjective (alta). It literally translates to "the mountain most high". Notice that la and alta both match the feminine singular noun montaña.

Can I change the word order and say "la más alta montaña"?

Usually, no. In Spanish, descriptive adjectives generally come after the noun. Because of this, the whole phrase más alta goes after the noun, giving us la montaña más alta.

Why does it use del mundo instead of en el mundo for "in the world"?

When talking about "the most" or "the least" of a certain group or place, Spanish uses the preposition de (of) rather than en (in). It literally means "the highest mountain of the world". Also, remember that de + el must combine into the single word del.

Why does the question start with Cuál instead of Qué?

When asking to pick one specific thing out of a group (like one mountain out of all the mountains), you use Cuál es. If you used Qué es, it would sound like you are asking for a dictionary definition, such as "What is a mountain?".