Final answer:
The monomial with the highest degree is 12a^(7).
Step-by-step explanation:
The monomial with the highest degree is 12a^(7) because the degree of a monomial is determined by the exponent of the variable. In this case, the variable 'a' has an exponent of 7 in the term '12a^(7)', which is greater than any other exponent in the given options.