Correct answer is (a) Xenobiotics
To explain I would say: Xenobiotics are the foreign substances which are chemical in nature found and within an organism which is produced naturally by the body or they may be present in excess amounts than usual, Bio-remediation is a process used to treat contaminated media using microbial methods, bio-fortification is a process which is used to increase the nutritional quality of crops, bio-leaching is a process used for extraction of metals.