Analysis of test questions: This question mainly examines students' ability to recognize and reproduce historical knowledge. It is required to understand the artistic characteristics and works of famous painters and calligraphers in Tang Dynasty. To solve this problem, we should first find the key word "great calligrapher", so we exclude C and D because they are both painters. Yan Zhenqing is the most accomplished calligrapher in the history of calligraphy in China after Wang Xizhi. The correct answer to this question is a.