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)