The answer is sterilization. Milady's definition is exact: "Sterilization..., which is the process that destroys all microbial life including spores." That "including spores" clause is the key distinction — spores are a dormant, especially hardy bacterial form that ordinary disinfecting doesn't reliably kill, which is why sterilization is a separate, higher-level term.
Cleaning, sanitizing, and disinfecting — the other three choices — form a real hierarchy: cleaning removes visible debris, sanitizing reduces germs to a safe level, and disinfecting kills most (not all) pathogens on surfaces. Sterilization sits above all three as the only one of the four that Milady defines as destroying spores specifically.
Milady Standard Barbering, 6th ed., Ch. 4 (Infection Control), p. 72: "Sterilization..., which is the process that destroys all microbial life including spores."
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