Final answer:
Protostars typically form in dark nebulae, dense and obscured regions full of gas and dust conducive to star formation. Hence, the correct answer is option C.
Step-by-step explanation:
Protostars are most likely to form in dark nebulae, which are dense regions of gas and dust where the conditions are optimal for the gravitational collapse of material to form new stars. These areas are often obscured from view because the dust blocks visible light, making them appear dark. However, stars are actively forming within these dense clouds.