SOLUTION
We are told that DJ was solving a linear equation
[tex]\begin{gathered} 18+7x-3=10x+15-3x \\ \text{which becomes } \\ 18-3+7x=10x-3x+15\text{ when he collects like terms, then } \\ 15+7x=7x+15 \\ \text{subtracting 15 from both sides } \\ 15-15+7x=7x+15-15 \\ 0+7x=7x+0 \\ 7x=7x \end{gathered}[/tex]
So, DJ's solution was correct.
Now since he got
[tex]7x=7x[/tex]
there are infinitely many solutions, because x can take any number.
Hence, there are infinite solutions since 7x = 7x
Therefore, the correct answer is option C