Final answer:
The error in the sentence is in the improper connection of two independent clauses using a comma instead of a semicolon before the coordinating conjunction. The corrected sentence should use a semicolon before 'but' since the two clauses are closely related in meaning.
Step-by-step explanation:
The sentence in question is: Nick Mason went to school to become an architect but his artistic dreams came true when he became the drummer for Pink Floyd. There is an issue with the structure of the sentence, particularly with how the two independent clauses are connected.
The correct approach should employ a semicolon to connect closely related independent clauses. Using a comma to connect independent clauses is incorrect, as it creates a comma splice which is a common grammatical error.
The correct answer is C. A semicolon should precede the coordinate conjunction.
Corrected sentence: Nick Mason went to school to become an architect; but his artistic dreams came true when he became the drummer for Pink Floyd.
Please note, this is a bit of an exceptional case since coordinating conjunctions typically do not follow semicolons, however when the independent clauses are closely related and balance each other, as in this sentence, it can be stylistically acceptable to use both together.