Which type of cocktail is served straight up?

The correct answer is tied to the definition of how a cocktail is served. Cocktails that are served "straight up" are those that are prepared by shaking or stirring the ingredients with ice to chill them, and then the mixture is strained into a glass without ice. This method ensures that the cocktail is served cold but without dilution from ice in the final presentation.

This means that when looking at other options, shaken cocktails with ice typically involve serving the drink with ice still in the glass, which does not qualify as "straight up." Blended cocktails, often served with ice, also do not meet this definition, as they usually maintain a slushy texture due to ice being incorporated. Frozen cocktails are also served in a way that includes ice or frozen elements, which again does not align with the "straight up" serving method. Thus, the unique aspect of cocktails served without ice in a chilled glass directly points to option C as the accurate choice.