Why does Ga improve conductivity of Ge?
This questions has two parts if you look closely:


  Which of the following elements is important in the semiconductor industry to improve the conductivity of Ge?


All of the above will improve the conductivity of $\ce{Ge}$.  This is because germanium is a semiconductor which conducts electricity via the movement of holes and electrons.  Since germanium is tetravalent and gallium is trivalent, doping germanium with gallium creates a p-type semiconductor which is exponentially rich in holes, thus increasing the conductivity. This mechanism holds true for aluminum, potassium and magnesium meaning all of them can increase the conductivity of germanium


  Which of the following elements is important in the semiconductor industry to improve the conductivity of Ge?


This part is what makes gallium special.  Gallium and germanium are sequential elements in atomic number.  They have very similar atomic radii.  Gallium is also very much less reactive than the other elements mentioned. These characteristics allow gallium to dope the germanium to a higher concentration without forming a new  undesired solid phase.  Sodium and magnesium form germanides with germanium and will not dope as shown by the phase diagram.  Aluminum will dope, but its smaller atomic radius can alter and strain the crystal structure of germanium much more than gallium which can affect band gap.