Marketers who design and offer new products and services to their existing customers are pursuing which growth strategy?

The correct answer is product development, as this strategy specifically involves creating new products or services to add to the current offerings for existing customers. This approach is aimed at meeting the evolving needs and preferences of those customers, thereby enhancing customer satisfaction and potentially increasing sales from the existing customer base.

By innovating or modifying products, companies can differentiate themselves in the market and leverage their existing customer relationships to drive growth. This strategy allows businesses to capitalize on their current market presence while minimizing risks associated with entering new markets.

The other options reflect different strategies that do not fit the scenario described. Market penetration focuses on increasing sales of existing products in existing markets, diversification involves creating new products for new markets, and market development entails entering new markets with existing products. Each of these strategies has its distinct objectives and implications, which highlight why product development is the most appropriate choice in this context.