Step 1: Understanding consumer-related problems
- Consumer-related problems refer to issues that consumers face in relation to products or services, such as pricing, measurement accuracy, and guarantees. These problems directly affect customer satisfaction and their buying decisions.
Step 2: Analyzing the options
- (A) Reasonable price: A reasonable price is a common concern for consumers, as they seek value for money and affordability.
- (B) Correct weight and measurement: Consumers expect products to be accurately measured and correctly weighed, making this an important consumer-related problem.
- (C) Lifetime guarantee of goods: Many consumers look for lifetime guarantees, which ensure that the product will be replaced or repaired if necessary, making it a consumer-related issue.
- (D) Quality product: Quality is a basic expectation for consumers, but it is not considered a problem if the product is of good quality. It is rather a standard expectation.
Step 3: Conclusion
Thus, the correct answer is (D) Quality product, as it is not a problem but rather an expectation in consumer transactions.
\[
\boxed{\text{Correct Answer: (D) Quality product}}
\]