Why D? And why not the others?

Correct answer: D. Yuri Gagarin

Soviet cosmonaut Yuri Gagarin became the first human in space on 12 April 1961, circling Earth once aboard Vostok 1 in a flight lasting just 108 minutes — and he parachuted separately from his capsule before it hit the ground. Neil Armstrong is the tempting mix-up because he was the first person to walk on the Moon, but that came eight years later, in 1969. Alan Shepard became the first American in space only weeks after Gagarin, on a short suborbital hop, and John Glenn was the first American to actually orbit Earth, in 1962.

Source: NASA: First Human Entered Space, April 1961 (nasa.gov/image-article/april-1961-first-human-entered-space/)