A logarithmic function without a subscript means that it must be log base 10.
![\log_(10)987=a](https://img.qammunity.org/2020/formulas/mathematics/middle-school/vw12vqve2flhb3s0re4af3dzekw1897xts.png)
When learning about logarithms, a textbook/teacher provides their students with the following general equations
![\log_yx=z](https://img.qammunity.org/2020/formulas/mathematics/middle-school/wlak1v0kory8dxq21627rf948e1n0mz5vu.png)
![y^z=x](https://img.qammunity.org/2020/formulas/mathematics/middle-school/y0z4un3cxxu6qoeiw2xq77c4cnoktqb20f.png)
Hopefully you are familiar with this. Using the equations that I just showed, we can change the equation
to
![10^a=987](https://img.qammunity.org/2020/formulas/mathematics/middle-school/ft6vh1ppityicyol7oq009lkt09doevcsc.png)
This would make answer choice C correct.