The Mach number after a shockwave is
inversely proportional to the mach number before it. ie. the
faster the mach number before a shock wave the slower it will be after it.
Looking at the options
a)Lower than 1 - This is not possible as
then we'd not have supersonic flow in the first place
b) exactly 1
- if the flow is sonic then shockwaves have not yet formed
c) high
(supersonic) - this will give a much slower mach number after the shockwave
d) small,
but supersonic, this will therefore give you a lower mach number after the
shockwave, but not as slow as that formed in (c)