A screw with two separate thread starts wound side by side is called a "double threaded" screw, so in one full turn of the screw it does not just move forward by one pitch length, it moves forward by two pitch lengths, since both threads advance together. That is exactly what "lead" means, how far the screw travels along its axis in a single rotation. Here the pitch is given as \( 2\text{ cm} \), and since it is double threaded, the screw advances twice that in one turn, giving a lead of \( 2 \times 2\text{ cm} = 4\text{ cm} \). The mean diameter of \( 8\text{ cm} \) mentioned in the question does not affect this calculation at all, it would only matter if we were finding torque or efficiency of the screw. So the lead is \( 4\text{ cm} \), which is option (D).