Step 1: Understand the types of sales promotion.
Sales promotions include various techniques to boost product sales, such as advertisements, personal selling, public relations, and sales events.
Step 2: Compare the options.
- Personal selling is typically the costliest because it involves direct interaction with customers, requiring trained sales staff and often considerable time and resources.
- Advertising, sales promotions, and public relations can be costly as well, but personal selling usually incurs the highest costs per transaction.
Step 3: Conclude.
Personal selling requires more resources and time, making it the costliest means of sales promotion.
Final Answer:
\[
\boxed{\text{Personal selling}}
\]