Which equation correctly expresses the definition of a logarithm ${\log }_b\left(a\right)=c$?

If ${\log }_5\left(125\right)=c$, what is the value of $c$ based on the definition of logarithm?

Condense the sum ${\log }_2\left(5\right)+{\log }_2\left(8\right)$ into a single logarithm.

Expand: ${\log }_{10}\left(20x^2{y}^3\right)$

Condense into one logarithm and simplify: $\frac{1}{2}{\log }_2\left(8\right)+{\log }_2\left(5\right)-3{\log }_2\left(2\right)$