When we say "survival of the fittest", does it mean that a. those which are fit only survive, or b. those that survive are called fit? Comment.

Survival of the fittest means those who survive are fit.

In the struggle for existence, the individuals who have more favourable variations will enjoy an advantage over others. They are fit and survive and reproduce.

Natural Selection – is a process in which heritable variations enabling better survival are enabled to reproduce and leave greater number of progeny. That is nature selects only those traits which help an organism to survive in the changed conditions. Those that are not fit for survival perish in the long run leading to evolution of new species.