When does wake turbulence begin for a departing large aeroplane?

Wake turbulence is a phenomenon created by an aircraft as it generates lift, particularly from the wingtip vortices produced during flight. For a departing large aircraft, the wake turbulence begins during the critical phase of flight known as rotation. This is the moment when the aircraft's nose lifts off the ground, and it begins to gain altitude.

At this point, the aircraft is generating significant lift, and thus, the vortices that contribute to wake turbulence are formed. These vortices can be particularly hazardous to smaller aircraft that may follow too closely behind a larger craft due to the strength and persistence of this turbulence.

In contrast, the other options do not capture the timing of when wake turbulence is generated for a departing large airplane. Touchdown corresponds to when an aircraft is landing, and while it does create turbulence, it's not relevant to a departing aircraft. The takeoff roll is when the aircraft is accelerating down the runway but has not yet reached the point of generating lift via rotation. Finally, the aircraft reaching altitude indicates that it is well beyond the wake turbulence phase associated with takeoff, as wake turbulence is primarily of concern during the initial phases of flight, particularly shortly after rotation.