Answer:
A. I broke my piano; I need to buy a new one.
Explanation:
Commas and semicolons can be used to connect different types of clauses.
Independent Clauses
There are 2 types of clauses in a sentence. Independent clauses are phrases that can stand alone as complete sentences. Dependent clauses are phrases that cannot stand alone. To be an independent clause there must be a subject, verb, and complete thought. Take, for example, the clause "I broke my piano." I is the subject, broke is the verb, and it conveys a complete thought. By this same process we can see the "I need to buy a new one" is also an independent clause.
Semicolons
Semicolons are used to connect 2 or more independent clauses. Commas cannot be used to connect independent clauses. This mistake is referred to as a comma splice. Commas should be used to connect dependent clauses to independent clauses. Since both the first and second clauses of the sentence are independent, we need to use a semicolon.