Here's why:
* Simple sentence: Contains one independent clause (a complete thought).
* Compound sentence: Contains two or more independent clauses joined by a coordinating conjunction (for, and, nor, but, or, yet, so).
* Complex sentence: Contains one independent clause and one or more dependent clauses.
The sentence "Farida had not comprehended how big the tree was until she stood at its base" has only one clause, and that clause is independent (it can stand alone as a complete thought).