Answer:
The noun clause is "that Stella took dance lessons at her age."
Explanation:
A noun clause is a subordinate/dependent clause, usually set off by a relative pronoun - that, which, who etc. A noun clause, as its name tips off, functions as a noun would, and it can act as the subject of a sentence or as the object of a verb or a preposition.
In the sentence "No one could believe that Stella took dance lessons at her age", "no one" is the subject and "could believe" are the verbs. The whole noun clause "that Stella took dance lessons at her age" functions as the object of the verb "believe". A simple trick would be to substitute the noun clause for a noun or a pronoun: "No one could believe it", or "No one could believe that fact."
