Final answer:
Improving a product may not be advisable when it is a signature item of a successful brand or when the product is obsolete. Other scenarios generally benefit from improvement, such as when facing competition or declining sales.
Step-by-step explanation:
Under certain circumstances, improving a product might not be a good idea. One such scenario is when the product is a signature product of a successful brand (option a). In this case, the brand's identity and customer loyalty might be tied to the current design, and changes could potentially alienate existing customers. Another situation where improvement may not be advisable is when a product is obsolete and unlikely to sell (option d); pouring resources into updating such a product may be wasteful, as the market may have moved on to newer and more relevant technologies or products.
However, it is generally beneficial for a business to improve its products when facing increased competition (option b), or when sales are beginning to decline (option c), as these improvements can attract new customers and revive product sales. Nonetheless, each decision to improve a product should consider the specific market dynamics and brand positioning involved.