Place value refers to the value of a digit based on its position in a number. In the given number 25.639, the digit 6 is in the tenths place. Let's break down the number and examine each digit's place value to understand why the correct answer is option C - 6 tenths.

1. Breaking down the number:
- The whole number part is 25, which is composed of the digit 2 in the tens place and the digit 5 in the units place.
- The decimal part is 0.639, which is composed of digits after the decimal point.

2. Understanding place value:
- In the whole number part, the digit 2 is in the tens place, which means it represents 20.
- The digit 5 is in the units place, which means it represents 5.

3. Focus on the decimal part:
- The digit 0 before the decimal point does not have a value in this case because it is in the ones place, and there are no ones in the decimal part.
- The digit 6 is in the tenths place, which means it represents 0.6. This is because the place value to the right of the decimal point is 1/10 (or 0.1), and the digit 6 is multiplied by this value.
- The digit 3 is in the hundredths place, which means it represents 0.03. This is because the place value to the right of the tenths place is 1/100 (or 0.01), and the digit 3 is multiplied by this value.
- The digit 9 is in the thousandths place, which means it represents 0.009. This is because the place value to the right of the hundredths place is 1/1000 (or 0.001), and the digit 9 is multiplied by this value.

4. Conclusion:
- The digit 6 in 25.639 is in the tenths place, representing 0.6.

Therefore, the correct answer is option C - 6 tenths.