Why do astronauts not 'feel' gravity? Stated another way, What is micro$-$gravity?

Question $31$ options:

a$)$

Because above the Karman Line, the gravitational effect of earth is absent

b$)$

Because their forward thrust is so great, they cannot feel the gravity

c$)$

Because they are in constant free fall, with nothing to push against that is not also in free fall

d$)$

None of these are true, because astronauts do feel gravity