Step 1 - Understanding quantum numbers
Quantum numbers are numbers that arise from the equation of quantum physics, and are very important in describing the electronic configuration of an atom, i.e, how the electrons are arranged around the nucleus.
Step 2 - Which quantum number specifies the shape of the orbital?
The principal quantum number, n, specifies the most probable distance between the electron and the nucleus.
The azimutal quantum number (l) describes the shape of the orbital, i.e., if it's an s-, p-, d- or f-type orbital.
Answer: b. l