If turbulence penetration speed is not given in the flight manual then what speed would be used for turbulence penetration speed.

1.) Vno
2.) Va
3.) Vne
4.) Vso

I reckon it would be the speed at which full and abrupt control inputs would not cause structural damage

Ellzy is spot on, in the absence of a published V_b figure, V_a (manoeuvering speed) is a good approximation. V_a is the speed where full and abrupt control movements could cause excessive aerodynamic loads on the airframe. In turbulent conditions you can think of the rapidly changing direction of the relative airflow as acting like very sudden control inputs. So, using V_a is going to be a good approximation for V_b. As long as you are below V_a all but extremely severe turbulence is going to induce a stall before dangerous aerodynamic loads occur.

Cheers,

Rich