The correct option is
C sin(P+Q)∣∣→OX×→OY∣∣=∣∣→OX∣∣∣∣→OY∣∣sinθ, where
θ is the angle between
→OX and
→OY.
∣∣→OX∣∣=∣∣→OY∣∣=1 since they are both unit vectors.
Also since →OX∥→QR and →OY∥→RP, θ is also the angle between →QR and →RP, i.e., ∠R
∴∣∣→OX×→OY∣∣=1.1.sinR
Now, since PQR is a triangle, P+Q+R=π⇒R=π−(P+Q)
∴∣∣→OX×→OY∣∣ =sin(π−(P+Q))=sin(P+Q)
Hence, option D is correct.