Вставьте пропущенное слово: The most important thing in marketing is finding out ___ the customers are and what they want.

The sentence is a statement about the core of marketing. The missing word should logically complete the phrase "finding out ___ the customers are". The most fitting word to describe what is important to find out about customers in marketing is their needs, desires, or identity.

Considering the context of marketing, understanding who the customers are and what they want is fundamental.

Therefore, the complete sentence would be: "The most important thing in marketing is finding out who the customers are and what they want."

Answer: who