Step 1 - Recall the Arrhenius equation.
$$k = A\,e^{-E_a/RT}$$
where $A$ is the frequency (pre-exponential) factor and $E_a$ the activation energy.

Step 2 - Interpret each factor physically.
$A$ = total number of collisions per second per unit volume.
$e^{-E_a/RT}$ = the FRACTION of those collisions in which the molecules possess energy
equal to or greater than $E_a$.

Step 3 - Link to the Maxwell-Boltzmann distribution.
From the Maxwell-Boltzmann distribution of molecular energies, the fraction of molecules
whose energy exceeds a threshold $E_a$ is
$$x = \frac{N_{E_a}}{N_T} = e^{-E_a/RT}$$

Step 4 - Check the other options.
$+\frac{E_a}{RT}$ and $-\frac{E_a}{RT}$ are only exponents, not fractions.
$e^{+E_a/RT}$ is greater than 1 for positive $E_a$, and a fraction can never exceed 1.

Answer: (B) $e^{-E_a/RT}$