Final answer:
Diva Boutique should adopt an in-house advertising agency to maintain a consistent brand image and have tighter control over advertising and promotional activities, tailored to their specific branding needs.
Step-by-step explanation:
The Diva Boutique in Manhattan, currently expanding and concerned with maintaining a consistent brand image, would benefit most from establishing an in-house advertising agency. Unlike external agencies, an in-house agency allows a company to exercise tighter control over the creative process and ensures that the brand's narrative remains cohesive across all advertising and promotional activities. This internal team can focus exclusively on Diva Boutique's branding, creating a unified message that correlates with the company's values and vision, reflecting a level of synergy similar to the integrated branding efforts described in Naomi Klein's 'No Logo' and the example of Miller beer advertising.