To express the fraction \(\frac{21}{64}\) as a decimal fraction rounded to 4 decimal places, follow these steps:
1. Divide the numerator by the denominator:
- Numerator is 21.
- Denominator is 64.
- Perform the division: \(21 \div 64 \approx 0.328125\).
2. Round the resulting decimal to 4 decimal places:
- The decimal fraction obtained from the division is \(0.328125\).
- When rounding \(0.328125\) to 4 decimal places, we look at the fifth decimal place.
- The fifth decimal place is a "2", which means we round down (since it is less than 5).
Thus, rounding \(0.328125\) to 4 decimal places results in \(0.3281\).
Therefore, the decimal fraction \(\frac{21}{64}\) rounded to 4 decimal places is:
[tex]\[
\boxed{0.3281}
\][/tex]
So, the correct answer is:
a. 0.3281
