Step 1: Recall the nature of alpha brass.
Alpha brass is nothing but a solid solution, zinc atoms substituting for copper atoms inside copper's FCC lattice, without forming any second phase. Like all solid solutions, it can only hold zinc up to a certain solubility limit before a new phase must appear.
Step 2: Locate that limit on the Cu-Zn diagram.
On the copper zinc equilibrium diagram, the alpha phase field is widest at around $456^\circ C$, where it can dissolve close to 39% zinc, but as the alloy cools further to room temperature this solubility boundary curves inward and settles at roughly 35 to 36% zinc.
Step 3: What happens beyond the limit.
Add more zinc than this, and the excess cannot stay dissolved in the FCC copper lattice, so a second, harder and more brittle beta phase begins to appear alongside alpha, giving the so called alpha plus beta or duplex brasses used for hot working.
Step 4: Selecting the range.
Since single phase alpha brass exists continuously from pure copper up to this solubility ceiling, its zinc content spans 0% to about 36%, which rules out the higher ranges that describe beta or gamma brasses.
\[ \boxed{0 - 36} \]