Final answer:
Option A, 25x^2y, is the monomial among the given choices because it is the only expression that consists of a single term.
Step-by-step explanation:
The question asks which of the provided expressions is a monomial. A monomial is a mathematical expression that contains only one term. It can be a constant, a variable, or a product of constants and variables raised to non-negative integer powers. Out of the options given, A. 25x^2y is a monomial because it consists of a single term with variables raised to non-negative integer powers. Options B, C, and D are not monomials because they contain more than one term, making them polynomials or sums of multiple terms.