In the presented situation, are you obliged only to turn right?

Why?

The blue C-8 sign requires you to turn either right or left, so both turns are permitted. Sign D-4b below it tells you that the road on the left is a dead end, but does not prohibit entry. You may therefore turn left as well, while continuing straight ahead is prohibited.

§ 35 ust. 1 pkt 8 i ust. 2 oraz § 45 ust. 2 rozporządzenia w sprawie znaków i sygnałów drogowych