Answer (a)
A transition altitude is normally specified for each aerodrome by the State in which the aerodrome is located. It is the altitude below which the vertical position of an aircraft is controlled reference to altitudes. The height above the aerodrome of the transition altitude shall be as low as possible but normally not less than 900m (3000').
Flight level zero is located at the atmospheric pressure level of 1013.25hPa.
Hope this helps,
Ruthless