c is fixed and defined for the speed of light in a vacuum.
c is a universal constant and does not vary.
However, to an outside observer, light will appear to travel slower through
other mediums (like air or glass). But really what is happening is the
wavelengths are being stretched, and it is taking more time to travel, but c
remains the same, relative to the light wave itself.
Light cannot exist unless it is travelling at c. It has no mass and only
one speed.
On the other hand, any real object (with nonzero mass) cannot reach c,
because as it approaches c, it's relativistic mass must approach infinity.
Bonus round:
The stretching of wavelengths mentioned above is the cause of prisms and
rainbows. If you take blue light and stretch it, it becomes red.
Light rays (like in lenses) will always take the path of least TIME,
regardless of mediums.
But time does not pass for things at the speed of light. For a light wave
zooming around the universe, it interacts with the entire universe at the
same moment, from its point of view. Chew on that...