An electron gun releases 2.0 × 10^ 16 electrons per second....

Electric current is defined as the rate of flow of electric charge.

$I = \dfrac{q}{t}$

Since $q = ne$, where $n$ is the number of electrons and $e$ is the elementary charge, the current can be written as:

$I = \dfrac{n}{t} \times e$

Given the number of electrons released per second is $\dfrac{n}{t} = 2.0 \times 10^{16} \text{ s}^{-1}$ and the charge of an electron is $e = 1.6 \times 10^{-19} \text{ C}$.

$I = (2.0 \times 10^{16}) \times (1.6 \times 10^{-19}) \text{ A}$

$I = 3.2 \times 10^{-3} \text{ A}$