Auxins are a class of plant hormones primarily responsible for growth regulation. The most common group in auxins is the carboxyl group (-COOH). However, the correct answer in this case points out that the "COOH" group represents the essential part in auxins, which likely refers to their carboxyl group attachment. Hence, the group associated with auxins is the carboxyl group (-COOH), which appears in the option as COOH.