Series of six to nine consecutive histidine residues can bind to immobilized metals such as nickel (ni). the purification of a protein containing six histidines at its c-terminus using a nickel-based resin would be achieved by chromatography.

cation exchange

Explanation:

Based on the information provided within the question it can be said that this would most likely be achieved by cation exchange chromatography. This is mainly due to the fact that Nickel (Ni) is a cation and that Histidine contains an imidazole side chain that is positively charged. Therefore since this is using a nickel based resin then it will be best achieved through cation exchange chromatography.

answer; i believe the correct answer is (crops);