Explanation:

✅ Option D: A custom attribute of type enum-of-string (single selectable value)

Since the products are categorized into three distinct types (boots, sandals, sneakers), and the business requirements specify that the messaging on the Product Detail page is different for each type and that customers should be able to filter their Product Search Results by product type, an enum-of-string attribute with a single selectable value is the best choice. This allows the customer to select one value for each product, which makes it easier to differentiate between boots, sandals, and sneakers, both for displaying different messages and for filtering search results.

Why "enum-of-string (single selectable value)" works:
1. It provides a predefined set of options (boots, sandals, sneakers), ensuring consistent categorization.
2. Since only one product type is applicable at a time, the single-select nature of the enum is appropriate.
3. This format also allows easy filtering for customers, as search filters typically rely on a single, well-defined value.

❌ Option A: A custom attribute of type string containing comma separated values.

This option would store the product types as a comma-separated string (e.g., "boots, sandals, sneakers"), which is not ideal. It complicates filtering and searching because the system would need to parse the string to check for specific values, and multiple types might be included in a single product, which conflicts with the business requirement of having one type per product. This would also make it more challenging to apply the required custom messaging per product type.

❌ Option B: A custom attribute type set-of-string containing multiple values.

While a set-of-strings allows multiple values, this is not suitable here because the product is restricted to only one type (boots, sandals, or sneakers). Using a set-of-string would allow customers to select multiple types, which is unnecessary and confusing for this particular scenario. The business requirement specifies that each product should only have one type, and filtering based on multiple types is not required.

❌ Option C: A custom attribute of type enum-of-string (multiselect able value).

A multiselect enum-of-string is inappropriate because it allows selecting multiple values for the product type, which conflicts with the business requirement that each product should belong to only one category (boots, sandals, or sneakers). Since a product can only have one type, a single-select enum is more fitting.