Which of the following color modes is best to use to print a poster at a professional printer?

CMYK is the best color mode to use for printing a poster at a professional printer because it is specifically designed for the printing process. CMYK stands for Cyan, Magenta, Yellow, and Key (Black), which are the primary colors of pigment used in color printing. This mode allows designers to create images that accurately reflect how the printed colors will appear on paper, as printers use these four ink colors to mix a wide spectrum of colors.

When preparing digital artwork for print, using CMYK helps to ensure that the colors will remain consistent and vibrant after the printing process. In contrast, RGB (Red, Green, Blue) is optimized for digital screens and may not translate directly to print, as it uses light rather than ink to display colors. Indexed color and Lab color modes serve different purposes and are less common for professional printing tasks. Indexed color is typically used to reduce the number of colors in an image to make it smaller in file size for certain applications, while Lab color is used for specific color adjustments and is not commonly used in standard print workflows. Therefore, when it comes to preparing designs for professional printing, CMYK is the preferred and most appropriate choice.