The correct answer option-1, "The federating units consented to form a union," is not a characteristic feature of Indian federalism.

Indian federalism is characterized by several features, which are worth noting. Firstly, Indian federalism is based on the consent of the federating units, meaning that the states voluntarily agreed to form a union and be a part of the Indian federal system. This consent is an essential aspect of federalism. However, option-1 states that the consent to form a union is not a characteristic feature, which is incorrect.

Regarding the other options, option-2 states that residuary powers vest with the Centre, which is a characteristic feature of Indian federalism. Residuary powers refer to those powers that are neither exclusively given to the states nor mentioned in the Constitution and thus are retained by the central government.

Option-3 states that Indian federalism has single citizenship, which is also an accurate characteristic feature. Indian citizens have one single citizenship, which is granted by the federal government.

Option-4 states that Indian federalism has an extensive Union and Concurrent list. This is also a correct characteristic feature of Indian federalism. The Union List includes subjects on which only the central government can legislate, while the Concurrent List contains subjects on