For the most effective strategies for mitigating climate change. Let's evaluate each statement:

The question asks for the most effective strategies for mitigating climate change. Let's evaluate each statement:

• a) It involves attempts to stabilise and reduce the human production of greenhouse gases. This is a primary and highly effective strategy for mitigating climate change, as greenhouse gas emissions are the main driver of global warming.
• b) It is associated with the erosion of the ozone layer. The erosion of the ozone layer is a separate environmental issue, primarily caused by ozone-depleting substances (like CFCs), not directly by climate change or its mitigation strategies. Therefore, this is not an effective strategy for mitigating climate change.
• c) It may include the establishment of carbon taxes. Carbon taxes are an economic tool designed to reduce greenhouse gas emissions by making activities that produce them more expensive, thereby incentivizing cleaner alternatives. This is an effective mitigation strategy.
• d) The abolition of international climate change agreements. International agreements (such as the Paris Agreement) are crucial for coordinating global efforts to mitigate climate change. Abolishing them would hinder, not help, mitigation efforts. Therefore, this is not an effective strategy; in fact, it is counterproductive.

Based on this analysis, statements 'a' and 'c' are effective strategies for mitigating climate change. Statements 'b' and 'd' are incorrect in this context.

Now, let's examine the given options:

1. a, b, c, and d
2. a, c, and d
3. a, b, and c
4. b, c, and d

The ideal answer would be "a and c". Since this option is not available, we must choose the "least incorrect" option among the choices provided.

• Options 1, 2, and 4 include statement 'd', which describes an action that would actively undermine climate change mitigation.
• Option 3 includes statements 'a' and 'c' (which are correct strategies) and statement 'b' (which is incorrect but less directly counterproductive than 'd').

Comparing the incorrect statements 'b' and 'd', 'd' represents an action that would actively work against climate change mitigation. Statement 'b' is factually incorrect in the context of mitigation strategies but does not propose a harmful action. Therefore, option 3, which contains the two correct strategies ('a' and 'c') and the less severely incorrect statement ('b'), is the most plausible choice.

The final answer is $\boxed{\text{3}}$