Answer:
a. Hydrogen
Step-by-step explanation:
Emission spectrum of hydrogen produces four main visible wavelengths out of which one is violet light at 412 nm.
Mercury has visible prominent lines of blue, green and yellow-orange.
Sodium has two prominent very close yellow wavelengths in its emission spectrum.
Neon spectra also consists of green to red lines but not violet light.
Thus, correct option is a.