Question

b electrons orbit in definite energy levels.
c doublets and triplets which appear in the spectra of some atoms.
d radiation can only occur when an electron jumps from one orbit to another orbit.
e (a), (b) and (d) are correct

Explanation:

In atomic physics, electrons orbit in definite energy levels (Bohr - model concept). Radiation is emitted or absorbed when an electron makes a transition between energy levels (orbits). Doublets and triplets in spectra are related to more complex quantum - mechanical effects beyond the simple Bohr - model assumptions and are not part of the basic tenets of the Bohr model which these statements seem to be related to.

Answer:

E. (a), (b) and (d) are correct