Final answer:
The true statement about standard cost systems is that favorable variances are not necessarily good, as they can arise from negative practices. Standard costs are also used for planning, with managers focusing only on significant variances, and typically, material price variances fall under the responsibility of the purchasing manager.
Step-by-step explanation:
The statement regarding standard cost systems that is true is: a. favorable variances are not necessarily good variances. This is because while a favorable variance indicates a cost or revenue that is better than the standard or expected amount, the reasons behind it might not be positive for the organization. For instance, a favorable variance might be due to cutting corners, reducing quality, or under-spending in essential areas, leading to long-term negative effects.
Managers typically do not investigate all variances from standard (b) — they focus on those that are significant and could impact business decisions. It is generally not the production supervisor but the purchasing manager who is responsible for material price variances (c). Finally, standard costs are indeed used for planning purposes (d), and adjustments can be made when costs change in the future. Thus, standard costs provide a framework for budgeting and monitoring financial performance over time.