Which statement best describes compatible numbers?

Compatible numbers are indeed numbers that are easy to calculate with mentally. This concept often involves selecting numbers that are close to the actual numbers in a calculation but have a simpler relationship, which simplifies the arithmetic involved. For example, when calculating 49 + 51, one might use 50 + 50, as both are easier to work with in one’s head and provide a quick estimation.

The other options do not accurately describe compatible numbers. While prime numbers are an important concept in mathematics, they do not inherently relate to the ease of calculations involving addition or subtraction. The idea of yielding zero when added doesn't apply to the broader definition of compatible numbers, which is not restricted to any specific arithmetic property. Lastly, compatible numbers can include fractions or decimals, not just whole numbers, which expands their application in various mathematical situations.