Option B:
is the correct answer.
Explanation:
The exponential equation is 
If
, then 
Thus, the equation becomes

Applying log rule,
and thus the equation becomes

Since, we know that,
, using this we get,

Hence, the logarithmic equation which is equivalent to the exponential equation
is 
Thus, Option B is the correct answer.
19 comes next in the series, followed by two 21s.
There appears to be a typing error in the second equation.
Still the answer is (7,10) because it is the only ordered pair that works for the first equation.