A rational number is any number that can be expressed as the quotient or fraction of two integers, where the denominator is not zero.
Let's evaluate each option:
A. \( 88\sqrt{} \) - This appears to be incomplete. If it means \( \sqrt{88} \), then it is an irrational number because \( 88 \) is not a perfect square.
B. \( 7\sqrt{} \) - Similar to the first option, if this means \( \sqrt{7} \), it is also an irrational number since \( 7 \) is not a perfect square.
C. \( 7.8 \) - This is a decimal number that can be expressed as a fraction (for example, \( \frac{78}{10} \)). Therefore, it is a rational number.
D. \( \pi \) - This is a well-known irrational number.
The correct answer is:
C. 7.8