In kingdom animalia, beginning of cephalisation is evident in phyllum:

Explanation

Cephalisation refers to the evolutionary trend of the development of a head region where nervous tissue becomes concentrated, forming a brain and sensory organs. This process is a significant step in the complexity of animal evolution. Among the given options, platyhelminthes, or flatworms, are the first group in the animal kingdom to exhibit this characteristic. They have a simple nervous system with a concentration of nerve cells forming a primitive brain and sensory organs at the anterior end, marking the beginning of cephalisation.