Final answer:
The problem that would NOT be faced by an unprotected Earthling stranded on Mars is strong gravity.
Step-by-step explanation:
The problem that would NOT be faced by an unprotected Earthling stranded on Mars is d) Strong gravity. While Mars does have weaker gravity compared to Earth, it is still strong enough to pose a challenge for astronauts. The other options, a) Low temperatures, b) Lack of oxygen, and c) Extreme radiation, are all significant problems that an unprotected Earthling would face on Mars.