"What's the price of this watch?"
Translation:¿Cuál es el precio de este reloj?
Here is a more thorough write-up of the choice between "qué" and "cuál". I would mention that at least in the English-from-Spanish course, you'll see "cuál" (as opposed to "qué") immediately before a noun. You'll also very likely see Spanish-speaking users complaining about this usage in the relevant discussion. My impression is that there is regional variation.
The important case of "¿Qué es esto/eso?" also doesn't clearly fit into the "look it up in a dictionary" guideline for when to use "qué," but I suppose if you expand the set of reference works to include Google Image look-up, it'd still work ;)
EDIT (addendum to first paragraph): The Real Academia Española officially says "cuál" is equivalent to "qué" before a noun, but that this usage is much more common in the Americas than in Spain.
I always equate "cuál" with "which," and then follow this logic:
Of the many prices that this watch could possibly have, WHICH ONE does it have?
In Mexico they often say, "Cuál es su nombre?" I always thought it was a strange turn of phrase until I thought, "There are millions of names in the world; WHICH of those millions is yours?"
Thinking this way helped me understand the usage. Every language has its own unique logic. Try to find it! I hope this helps.
This has been asked numerous times in different discussions. It is also explained by Duolingo under the Tips. When you click on a skill, click Tips for essential info and hints about Spanish. Click them and learn.
Este is masculine, esta is feminine. Esto is more esoteric. Learn more at https://forum.duolingo.com/comment/5388844/When-do-you-use-Esto-esta-este-eso-esa-ese-estos-estas