Step 1: Understanding the Concept
$e/m$ ratio represents the charge-to-mass ratio of a particle.
Step 2: Calculation for each
- **$He^{2+}$**: Charge $= +2$, Mass $\approx 4$ amu. Ratio $= 2/4 = 0.5$.
- **$H^{+}$**: Charge $= +1$, Mass $\approx 1$ amu. Ratio $= 1/1 = 1.0$.
- **$He^{+}$**: Charge $= +1$, Mass $\approx 4$ amu. Ratio $= 1/4 = 0.25$.
- **$D^{+}$** (Deuterium): Charge $= +1$, Mass $\approx 2$ amu. Ratio $= 1/2 = 0.5$.
Step 3: Answer Analysis
The $e/m$ ratio is highest for the proton ($H^+$).
Hence, the Answer is: (B)