1) The price of the boat is $7,500 more than the price of the trailer.
\(T = B + 7500\)
Multiple possible values
Insufficient \(\Large{\color{red}{\times}}\)

2) The combined price of the boat and trailer is $15,000.
\(B + T = 15000\)
Multiple possible values
Insufficient \(\Large{\color{red}{\times}}\)

Combining both :
Two equations, two variables (Solvable)
\(\begin{cases} T = B + 7500 \\ B + T = 15000 \end{cases} \Rightarrow B=3705,\, T=11250\)
Sufficient \(\color{green}{\checkmark}\)

Hence, __C__.