Which combination can lead to a firmer texture in whipped egg whites?

Adding cream of tartar to whipped egg whites is an effective method to achieve a firmer texture. Cream of tartar is an acid that stabilizes the egg whites by increasing the acidity of the mixture. This added acidity helps to strengthen the protein structures in the egg whites, allowing them to hold onto air better and resulting in a more stable foam. This stabilization is crucial when whipping egg whites, especially if they need to maintain their volume during mixing or baking processes.

In contrast, while whipping at high speeds can create volume, it may also lead to overwhipping, which causes the structure to break down, resulting in a less stable foam. Using cold egg whites could produce less volume initially, as egg whites whip better at room temperature. Adding milk would actually weigh down the foam and not contribute to the desired firmness in whipped egg whites. Thus, cream of tartar is the best choice for achieving that firmer texture.