Final answer:
The provided procedure does not match the student's question about reassembling a milkshake dispenser; therefore, it is not possible to determine the last step of the correct process without additional context.
Step-by-step explanation:
The last step in reassembling the valve block and plunger assembly of the milkshake base dispenser after monthly cleaning is not explicitly provided in the referenced procedure; however, based on the general practices of equipment maintenance and sanitization within the hospitality industry, priming the pump with sanitizer would be a logical final step. To ensure the safety and quality of the beverages served, it is important that all equipment is sanitized before use. The referenced procedure outlines steps concerning the cleaning of a plastic object rather than the reassembly of a milkshake dispenser, and therefore it is not possible to confidently provide the correct answer to the student’s question without the complete context of the milkshake machine's cleaning and reassembly process.