Final answer:
The letter in the middle '12 C 6' is 'C,' which is the symbol for Carbon. This is related to Chemistry and signifies the element in the notation where '12' is the mass number and '6' is the atomic number.
Step-by-step explanation:
The question pertains to Chemistry, and it's about understanding the symbols and numbers associated with an element. In the example provided, '12 C 6', 'C' represents the chemical symbol for Carbon, '12' represents the atomic mass number, and '6' represents the atomic number.
The letter in the middle, 'C', is the symbol for Carbon. Thus, the correct answer to the question 'What is the letter in the middle?' with respect to the given example '12 C 6' would be option B) C.