Step 1: Identify the key activities described in the case.
NV Electronics not only provides quality products but also handles customer complaints speedily, provides maintenance services, and arranges credit facilities for buyers.
Step 2: Understand the function of Branding.
Branding involves assigning a unique name, symbol, or design to a product to distinguish it from competitors. The case does not describe branding activities.
Step 3: Understand the function of Marketing Planning.
Marketing planning involves developing strategies and plans to achieve marketing goals. This is not what the case is describing.
Step 4: Understand the function of Product Designing and Development.
This function involves decisions about the features, shape, colour, and quality of the product itself. The case only mentions an existing product range, not its design decisions.
Step 5: Understand the function of Customer Support Services.
Customer support services include all after-sales activities like handling complaints, providing repair and maintenance, ensuring spare parts availability, and arranging credit services. These activities maximise customer satisfaction beyond the initial sale.
Step 6: Match and confirm.
Handling complaints speedily, maintenance services, and credit services are all post-sale activities that define Customer Support Services.
\[ \boxed{ \text{(D) Customer support services} } \]