Explanation:

NITI Aayog (National Institution for Transforming India) is a policy think tank of the Government of India, established to replace the Planning Commission. \nOption A, 'It's Chairperson is Prime Minister of India', is true. The Prime Minister of India serves as the ex-officio Chairperson of NITI Aayog. This structure ensures high-level political leadership and alignment with national priorities. \nOption B, 'All Chief Ministers of States and Lieutenant Governors of Union Territories are its members', is true. The Governing Council of NITI Aayog comprises the Chief Ministers of all States and Lieutenant Governors of Union Territories. This inclusive structure promotes cooperative federalism and ensures states' participation in policy formulation. \nOption C, 'It prepares Five Year plans for the country', is not true. This statement is false. NITI Aayog was established on January $1$, $2015$, specifically to replace the Planning Commission, which was responsible for formulating Five-Year Plans. NITI Aayog's mandate is to act as a 'think tank' providing strategic and technical advice to the central and state governments, focusing on long-term policy frameworks, programs, and initiatives, rather than preparing Five-Year Plans. Therefore, this is the statement that is 'not true' about NITI Aayog. \nOption D, 'Niti Aayog was established in 2015', is true. NITI Aayog was indeed established on January $1$, $2015$, through a resolution of the Union Cabinet, replacing the $65$-year-old Planning Commission.