Frequency is how many cycles occur each second, and the period is the time for one cycle. The period T is the reciprocal of the frequency f, so T = 1/f. For 25 Hz, T = 1/25 seconds, which equals 0.04 s. So each cycle lasts 0.04 seconds. The other numbers would correspond to different frequencies (for example, 0.25 s is a cycle time for 4 Hz, 4 s for 0.25 Hz, and 0.4 s for 2.5 Hz), not for 25 Hz.

The average value of a full-wave rectified sine is 2/π times the peak, which is about 0.636 of the peak. If you take v(t) = Vp sin θ and find the average of its absolute value over a full cycle, you compute (1/2π) ∫0 to 2π |Vp sin θ| dθ. The integral of |sin θ| over a full cycle is 4, so the average becomes (Vp/(2π)) × 4 = 2Vp/π ≈ 0.636 Vp. So the correct description is that the average voltage is 0.636 times the peak value.

In a balanced three-phase system, the total apparent power is found by summing the per-phase powers. Each phase has a voltage of V_phase and current I_phase, and if the load is balanced, all three phases contribute equally: S = 3 V_phase I_phase. For a star (Y) connection, the phase voltage is V_phase = V_LL / √3, and the line current equals the phase current, I_phase = I_L. Substituting gives S = 3 (V_LL / √3) I_L = √3 V_LL I_L. So the three-phase apparent power is the product of the line-to-line voltage, the line current, and √3. The other options don’t match this relationship: using 3 × V_LL × I_L would be too large by a factor of √3, √2 × V_LL × I_L introduces an unnecessary √2 factor, and V_LL × I_L alone represents a single-phase power, not the total three-phase power.

In a sinusoidal AC waveform, the RMS (effective) voltage is the peak voltage divided by sqrt(2). So the peak value is the RMS voltage times sqrt(2). For 100 V RMS, the peak is 100 × sqrt(2) ≈ 141.4 V. This is the maximum instantaneous voltage of the waveform. 70.7 V would be the RMS value corresponding to a 100 V peak, not the peak for a 100 V RMS. 100 V would imply the peak and RMS are the same, which isn’t true for a sine wave. 200 V would mean the peak is double the RMS, which also doesn’t fit a sine relationship.