Which of the following is a common technique for thickening sauces?

Reducing is a common technique for thickening sauces because it involves simmering or boiling a liquid to evaporate part of its water content. As the liquid reduces, the flavors become more concentrated and the sauce thickens due to the reduced volume. This technique is often used in culinary practices to enhance the flavor profile of sauces, making them more robust and appealing.

In contrast, blanching is primarily a cooking technique used to quickly cook vegetables in boiling water before plunging them into ice water to halt the cooking process. It is not designed to thicken sauces. Infusing refers to the process of steeping herbs or spices in a liquid to extract flavors, which does not inherently thicken a sauce. Emulsifying involves mixing two liquids that typically do not combine well, like oil and vinegar, to create a stable mixture, but this also does not serve the purpose of thickening.