(1S,2S)-1-Bromo-1,2-diphenylpropane is treated with $\mathrm{NaOEt}$/ethanol. The only beta hydrogen is on C-2, and the reaction is a concerted anti E2 elimination. The product is

In the conformation with H on C-2 anti-periplanar to Br on C-1, the two phenyl groups lie 60 degrees apart in the Newman projection, so after elimination they are cis. A three-dimensional model of the (1S,2S) isomer confirms this, giving the Z alkene, and the (1S,2R) diastereomer would give the E alkene instead. Anti elimination is stereospecific, so one isomer is formed.