Answer: Magnetic field.
- A Electric field
- B Magnetic field
- C Pressure
- D Temperature
Correct answer: B. Magnetic field
Explanation: Zeeman effect: spectral lines split in magnetic field due to interaction of orbital magnetic moment with field.

The Geiger–Marsden alpha-scattering result: a few alpha particles bounce back at large angles, which rules out Thomson’s diffuse model and demands a tiny, massive, positively charged nucleus. Image: Kurzon, CC BY 3.0, via Wikimedia Commons.
Concept context
Thomson's plum-pudding model, Rutherford's nuclear model from alpha-scattering, Bohr's postulates and hydrogen spectrum, spectral series (Lyman to Pfund), Rydberg formula, and limitations of each model.