Let's define each choice to differentiate which is the answer
A. Equivalent - equivalent equations may not look exactly the same on face value. But they are equivalent because they have the same exact solution.
B. Expressions - expression is a general term for equations that are formed from word problems
C. Equal - equal equations are the exact duplicate of each other
D. Similar - this term is only used on geometric shapes to tell that the two shapes have a fixed ratio of their similar sides or angles
E. Radical - radical equations are those involving fractions
Therefore, from their descriptions, the answer is A.