Step 1: Understanding the Concept:
Grapefruit (Citrus paradisi) cultivars are categorized into white-fleshed and colored (pink/red-fleshed) groups.
Many colored cultivars originated as natural mutations, specifically limb sports or budsports, arising on existing white-fleshed trees.
Step 2: Detailed Explanation:
Let us review the origin of the listed cultivars:
Duncan: It is an old, highly seedy, white-fleshed grapefruit variety, believed to be a seedling of the original grapefruit introduced to Florida.
Foster: It originated as a pink-fleshed budsport of the seedy white cultivar 'Walters' in 1907. It is seedy.
Thompson (also known as Pink Marsh): This variety originated as a natural pink-fleshed budsport of the seedless white-fleshed cultivar Marsh (Marsh Seedless) in 1913 in an orchard in Oneco, Florida.
It is seedless and represents the first seedless pink grapefruit variety.
Star Ruby: It was developed through physical mutagenesis (irradiation of Hudson grapefruit seeds) to achieve an intense deep-red flesh.
Hence, Thompson is the correct budsport of Marsh.
Step 3: Final Answer:
Since Thompson (Pink Marsh) is a budsport of Marsh, the correct option is (C).