The correct answer is 1/5 because when subtracting fractions, we need to have a common denominator. In this case, the common denominator is 10. So, 7/10 - 3/10 is equal to (7-3)/10, which simplifies to 4/10. To further simplify, we can divide both the numerator and denominator by their greatest common factor, which is 2. Thus, 4/10 simplifies to 2/5. Therefore, the correct answer is 1/5.