1. The problem asks to explain why the expression $\frac{4}{3} \pi r^3$ is a monomial and to identify its degree. 2. A monomial is an algebraic expression consisting of only one term, which is a product of constants and variables with non-negative integer exponents. 3. The expression $\frac{4}{3} \pi r^3$ has one term: the constant $\frac{4}{3} \pi$ multiplied by the variable $r$ raised to the power 3. 4. Since there is only one term, it is a monomial. 5. The degree of a monomial is the sum of the exponents of the variables in the term. 6. Here, the only variable is $r$ with exponent 3, so the degree is 3. Final answer: The expression $\frac{4}{3} \pi r^3$ is a monomial of degree 3.