A) 1/2: This is a fraction and represents a rational number. It is not an expression because it is a single value.

B) 3: This is a whole number and represents an integer. It is not an expression because it is a single value.

C) 3x-2: This is an expression. It consists of variables, constants, and operators. In this expression, "x" represents a variable and "3" and "2" are constants. The "-" is the subtraction operator. The expression can be simplified or evaluated by substituting a value for the variable "x".

D) 2: This is a whole number and represents an integer. It is not an expression because it is a single value.

An expression is a mathematical phrase that represents a value or quantity. It can consist of variables, constants, and operators. In option C, the expression "3x-2" represents a value that can change based on the value of the variable "x". It can be used to calculate different values depending on the value of "x".

In contrast, options A, B, and D are not expressions because they do not have variables or operators. They represent single values that do not change. Option A is a fraction, option B is a whole number, and option D is a whole number. These values do not depend on any variables or operators.

Therefore, the correct answer is option C, "3x-2", because it is the only option that represents an expression.