Which of the following can be used as a dessert sauce?

The choice of pastry cream thinned with heavy cream or milk as a dessert sauce is correct because pastry cream serves as a rich and flavorful base that can be easily modified to achieve a pourable consistency. By adding heavy cream or milk, you create a versatile sauce that can enhance a variety of desserts. This approach allows for customization in flavor and texture, making it suitable for drizzling over cakes, tarts, or other pastries.

The other options are limited in their application as dessert sauces. Chocolate syrup, while a classic choice, does not provide the same rich and complex flavor profile as a thinned pastry cream, as it is specifically one-note and focused on chocolate flavor without the creaminess of pastry cream. Buttercream icing, on the other hand, is traditionally used for frosting and decorating rather than serving as a sauce, as it is too thick and sweet for that purpose. Lastly, flavored whipped cream can function as a light topping but lacks the richness and body needed to be categorized effectively as a sauce on its own. Thus, the choice of thinning pastry cream is the most versatile and suitable for use as a dessert sauce.