First, we pay attention to the numerical coefficients of the terms in the series: 10, 21, 32, 43, 54, 65. Conclusively they form an arithmetic sequence with a common difference of 11. Thus, the next numerical coefficient is 76. Then, we pay attention to the letters which are just arrange alphabetically. The next letter ought to be G which needs to be capitalized. Thus, the answer is letter C. 76G.
