Option A is correct. According to the textbook context, 'Bioremediation is thus a process through which such toxic pesticides can be removed completely or its chemical nature can be altered.' This confirms the assertion is true. The reason is also true as the context states: 'In the process of bioremediation, the ability of microorganisms to degrade or alter the toxic nature of chemical pesticides into non-toxic substances is exploited.' The reason correctly explains the assertion because microorganisms are the biological agents that carry out the bioremediation process - they possess enzymes capable of degrading pesticides and can derive energy from these chemicals, thereby enabling complete removal or alteration of toxic pesticides.