Final answer:
Customer loyalty is the strategy focused on sustaining a competitive advantage by creating a loyal customer base that provides stable revenue and can deter competitors.
Step-by-step explanation:
Among the strategies listed, C. customer loyalty is specifically focused on sustainment of a competitive advantage. While cost leadership and cost focus strategies are about minimizing expenses, and differentiation is about creating a unique product or service, customer loyalty directly relates to maintaining a strong, ongoing relationship with customers. Businesses that successfully develop customer loyalty can create barriers to entry for competitors, ensure a stable revenue stream, and leverage their satisfied customer base for word-of-mouth marketing, all of which help to sustain their competitive advantage over time.