Pranayama, a Sanskrit word meaning "extension of the breath," is a practice in yoga that involves various breathing techniques. It is an essential part of yoga and is believed to have numerous physical, mental, and spiritual benefits. In this question, we are asked to identify which option is not a form of Pranayama. Let's analyze each option:

a) Kumbhak: Kumbhak is a type of Pranayama that involves holding the breath. It is characterized by inhaling, exhaling, and holding the breath in different patterns. Kumbhak helps to improve lung capacity, increase oxygen intake, and bring balance to the body and mind.

b) Ahimsa: Ahimsa, meaning "non-violence" in Sanskrit, is one of the foundational principles of yoga. It refers to the practice of non-violence towards oneself and others. While Ahimsa is not a specific Pranayama technique, it is an important ethical guideline that yogis follow in their yoga practice and daily lives.

c) Shatkarma: Shatkarma refers to a set of six cleansing techniques in yoga. These practices are intended to purify the body and mind by eliminating toxins and blockages. Shatkarma techniques include practices like Neti (nasal cleansing), Dhauti (cleansing of the digestive tract), and Trataka (gazing meditation). Although Shatkarma is not exclusively focused on Pranayama, it is often included as a preparatory practice to enhance the effectiveness of Pranayama.

d) Ujjayi: Ujjayi is a specific type of Pranayama that involves breathing with a slight constriction of the throat, creating a soft hissing or ocean-like sound during both inhalation and exhalation. This technique helps to increase breath awareness, calm the mind, and generate heat in the body. Ujjayi is commonly practiced in various styles of yoga, such as Ashtanga and Vinyasa.

Therefore, the correct answer is option 'D' - Ujjayi. Ujjayi is indeed a form of Pranayama, and the other options listed (Kumbhak, Ahimsa, and Shatkarma) are also related to Pranayama in different ways.