To find the relative frequency of a non-defective unit from the batch of calculators, we first need to determine the number of non-defective units.
If there are 500 units in total and 25 of them have non-functioning keys, then the number of non-defective units is:
\[ 500 - 25 = 475 \]
Next, we can express the relative frequency of a non-defective unit as the number of non-defective units divided by the total number of units:
\[ \text{Relative Frequency} = \frac{475}{500} \]
To simplify this fraction, we can divide both the numerator and the denominator by their greatest common divisor (GCD), which is 25:
\[ \frac{475 \div 25}{500 \div 25} = \frac{19}{20} \]
Thus, the relative frequency of a non-defective unit from this batch expressed in simplest form is:
\[ \frac{19}{20} \]
So the correct response is:
1920 Start Fraction 19 over 20 End Fraction