After how many hours will 300 colonies be present?

Substitute the given number in your formula and solve for the remaining variable.
.. 300 = 100e^(0.7t)
.. 3 = e^(0.7t) . . . . . . . . . divide by 100
.. ln(3) = 0.7t . . . . . . . . . .take the natural logarithm
.. ln(3)/0.7 = t . . . . . . . . . divide by the coefficient of the variable
.. 1.6 = t

Selection B is appropriate.