Final answer:
The correct statement that describes a problem encountered by national advertisers when using spot advertising is D. Spot advertising is usually characterized by significant variations in pricing and discount policies.
Step-by-step explanation:
Spot advertising refers to the process of buying ad spots on television or radio stations in specific local markets. This allows national advertisers to target their ads to specific regions. However, one major problem with spot advertising is the significant variations in pricing and discount policies.
For example, the cost of buying ad spots in popular markets or during peak times may be much higher compared to less popular markets or off-peak times. Additionally, different media outlets may offer different discount policies or pricing structures, making it challenging for national advertisers to plan and budget their advertising campaigns effectively.