Right answer is (b) .270
The best I can explain: Sinusoidal waveform is generally expressed in the form of V=Vmsin(ωt+α) where Vm represents peak value, ω represents angular frequency, α represents a phase difference. By comparing the waveform z(t) with the generalized sinusoidal expression we can see Vm=.27 and ω=.369π rad/s.