Final answer:
Red is typically considered to be a color with a warm undertone, associated with warmth and sunlight in color theory.
Step-by-step explanation:
Among the options listed, a warm undertone is typically associated with colors like red and yellow. So, the correct answer regarding warm undertones would be a. red, as it is commonly recognized as having a warm undertone in color theory. Conversely, yellowish-green can be perceived as more of a cool undertone due to its green component. Warm undertones in colors are often described as cozy, earthy, and reminiscent of sunlight and warmth.