Learn French: Quelle est la raison de ce retard ? - What is the reason for this delay?

Prepositions often don't translate word-for-word. While English says "the reason for", French pairs the word raison with de (of). So, you are literally asking "the reason of this delay". Save pour for when you are expressing a purpose or saying what an action is intended for!

Why do we use Quelle instead of Quel?

Quelle is the feminine form of "what" or "which". It has to match the gender of the noun it is asking about. Here, it refers to la raison (the reason), which is a feminine word. If you were asking about a masculine word, you would use Quel.

What does ce mean before retard?

Ce means "this" or "that". It is used to point out something specific. Because retard (delay) is a masculine noun, we use ce. If the noun were feminine, you would use cette instead (for example, cette raison meaning "this reason").

I've learned that "late" is en retard. How does that fit here?

Retard is actually a noun that means "delay" or "lateness". When you say someone is late in French using être en retard, you are literally saying they are "in delay". In this sentence, it is just used as a normal noun: ce retard (this delay).