The Ergonomic Society conducted a study that indicated that many people develop severe back problems during adulthood, and that virtually all such people who received chiropractic treatment showed great improvement. Therefore, in order to minimize the proportion of the population that suffers from back pain, the Ergonomic Society recommended that chiropractic treatment be directed toward those adults who suffer from severe back problems.

Which of the following is an assumption on which the argument depends?
Any person who receives chiropractic treatment for back pain may also benefit from other forms of treatment.
Large insurance carriers cover chiropractic care for back problems to a lesser degree than they do other medical treatments.
Individuals who receive chiropractic or other treatment prior to developing severe back problems are not less likely to develop back pain than those who do not.
Chiropractic treatment is more effective in treating severe back problems when utilized over a long period of time, as opposed to sporadically.
Severe back pain and other problems often cause individuals to miss workdays.

In the above answer choices, can someone apply Logical Negation to choice (C), which is the answer to the question.
My question is to know in which part of the sentence we need to apply the logical negation. In answer choice (C), we can apply logical negation at 3 places. So can someone explain where to apply.

Individuals who DO/DON'T receive chiropractic or other treatment prior to developing severe back problems are LESS LIKELY not less likely to develop back pain than those who do DOES not.

Thanks