To determine which of the following numbers is a rational number, let's review each option:
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\( 7 - \sqrt{7} \): This number is irrational because it involves the square root of 7, which is irrational.
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\( 7.8 \): This number is a rational number because it can be expressed as the fraction \( \frac{78}{10} \).
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\( 7.9837377283... \): This number appears to be a non-repeating decimal, therefore it is also irrational.
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\( \pi \): This is a well-known irrational number.
Based on the options, the rational number is 7.8.