ANSWER
[tex]\log _{}5^x\text{ = log21}[/tex]EXPLANATION
We want to find the very first step in solving the problem below:
[tex]5^x\text{ = 21}[/tex]If we wanted to jump steps, we could just state that the equation would become:
[tex]\log _521\text{ = x}[/tex]But that would be jumping a lot of steps and it is not among the options.
So, the very first step in solving this problem is to find the logarithm of both sides.
That is:
[tex]\log 5^x\text{ = log21}[/tex]That is the answer. (Option B)