Final answer:
Japanese thermal straightening combines the use of a thio relaxer with flat ironing to achieve straight hair.
Step-by-step explanation:
The technique that combines the use of a thio relaxer with flat ironing is C) Japanese thermal straightening. This process involves applying a thio relaxer to break down the disulfide bonds in the hair, making it more pliable, and then using a flat iron to straighten the hair. Japanese thermal straightening is popular for achieving long-lasting straight hair.