Why Power in a resistive load is P = V^2/R with V in RMS volts, so rearranged V = sqrt(P x R). Here sqrt(1200 W x 50 ohms) = sqrt(60,000) = about 245 volts RMS. A dummy load is purely resistive, so no reactance or power factor correction is needed.
Watch out The 346 volt choice is the peak voltage (245 x 1.414), and 692 volts is the peak-to-peak value; the question asks for RMS. The 173 volt figure is what you get if you wrongly divide the RMS answer by 1.414.
V(RMS) = sqrt(P x R). 1200 x 50 = 60,000, and sqrt(60,000) is about 245.