Answer:
To rewrite the radical expression "the seventh root of x to the thrid power" as an expression with a rational exponent, we need to use the property that allows us to rewrite a root of a power as a power with a rational exponent. This property states that "the nth root of x to the mth power" is equal to "x to the m/nth power".
Using this property, we can rewrite "the seventh root of x to the thrid power" as "x to the 3/7th power". Therefore, the correct answer is "x to the 3/7th power".
The other answer choices are not equivalent to the original expression. "x to the seven thirds power" is not a valid expression because "thirds" is not a unit of measurement. "x^21" is not equivalent to the original expression because the base and exponent are not related in the same way as in the original expression. "x^4" is not equivalent to the original expression because the base is the same but the exponent is not.