Which one of the following countries is not a member of the ”Quadrilateral Security Dialogue”, also known as ”QUAD”?

The correct option is China.

The Quadrilateral Security Dialogue, commonly referred to as the QUAD, is an informal strategic forum comprising four nations:
1. United States
2. India
3. Japan
4. Australia

The forum was maintained by talks between member countries and was initiated in 2007 by Prime Minister Shinzo Abe of Japan. The primary objective of the QUAD is to support a free, open, and prosperous Indo-Pacific region.

Among the given options, India, Japan, and Australia are active members of the QUAD, whereas China is not a member. In fact, the partnership is often viewed as a strategic counterweight to China's growing economic and military influence in the Indo-Pacific region.