Final answer:
The expression (-8z^2) is already simplified, so the correct answer is -8z^2, option (a).
Step-by-step explanation:
The expression (-8z^2) is already in its simplest form and does not need any further simplification. Therefore, the simplified form is -8z^2. When simplifying algebraic expressions, we typically look for like terms to combine or operations to perform, but in this case, there are no like terms to combine and no operations to perform on the single term provided.