The equivalent expression is log base 7 of 2401, making choice (C) the correct answer.
To simplify the expression log base 7 of 7 plus log base 7 of 343, we can use logarithmic properties. The first term, log base 7 of 7, is equal to 1, as any logarithm to the base of its own number is 1. The second term, log base 7 of 343, can be simplified by recognizing that 343 is 7^3. Therefore, log base 7 of 343 is equivalent to 3.
Now, combining both terms, we have log base 7 of 7 plus log base 7 of 343 equals 1 plus 3, which is 4. So, the given expression is equivalent to 4.
Now, let's examine the answer choices:
A. log base 7 of 49: This is equivalent to 2 because 7^2 equals 49. Not the same as the simplified expression.
B. log base 7 of 350: This doesn't simplify to the given expression.
C. log base 7 of 2401: This is equivalent to 4 because 7^4 equals 2401. It matches the simplified expression.
D. log 2401: The base is missing, making it ambiguous.
Therefore, the correct answer is (C) log base 7 of 2401, which is equivalent to the given expression log base 7 of 7 plus log base 7 of 343.