Larissa's gross monthly pay is determined by the annual salary divided by the number of months in a year.
Given that her annual salary is $45,000, we can calculate her gross monthly pay as follows:
\[ \text{Gross monthly pay} = \frac{\text{Annual salary}}{12} \]
\[ \text{Gross monthly pay} = \frac{45,000}{12} = 3,750 \]
Therefore, the correct answer is:
$3,750.00