Answer:
The correct option is A.
Step-by-step explanation:
The number of bacteria in an experiment can be represented by an exponential function

where, x represents the number of hours.
We need to find the value of x when f(x)=64.
Substitute f(x)=64 in the given function.

It can be rewritten as

On comparing both sides by get

The value of x is 3.
Since f(3)=64 it means the population of bacteria is 64 in 3rd hour.
Therefore the correct option is A.