The correct option is C angular momentum
As we know,
→τ=d→Ldt
where →τ= torque
−→dL= change in angular momentum
If →τ=0,d→Ldt=0
Hence, →L= constant
So, if net torque on the system about an axis is zero, then angular momentum of that system about that axis is conserved.
Hence, option (c) is correct.