Which of the following asana is not practiced in standing position?

The correct option is Shalabhasana.

To understand why this is the correct choice, let us examine the practicing positions for each of the listed yoga postures:
1. Trikonasana (Triangle Pose): This pose is performed by standing with the feet wide apart and stretching sideways. It is a classic standing yoga posture.
2. Tadasana (Mountain Pose): This is the basic starting posture for all standing poses, where the practitioner stands straight and steady with feet together.
3. Vrikshasana (Tree Pose): This is a balancing posture performed while standing on one leg with the sole of the other foot placed on the inner thigh.
4. Shalabhasana (Locust Pose): This posture is performed by lying flat on the stomach (in a prone position) and lifting the legs and upper body off the ground.

Since Shalabhasana is performed while lying down on the abdomen, it is the only posture among the options that is not practiced in a standing position.