A logarithmic function without a subscript means that it must be log base 10.
[tex]\log_{10}987=a[/tex]
When learning about logarithms, a textbook/teacher provides their students with the following general equations
[tex]\log_yx=z[/tex]
[tex]y^z=x[/tex]
Hopefully you are familiar with this. Using the equations that I just showed, we can change the equation [tex]\log_{10}987=a[/tex] to [tex]10^a=987[/tex]
This would make answer choice C correct.
