Right answer because:
We want to have a positive benefit on the environments and communities where we work.
That's why we establish environmental policies and continuously try to improve our environmental performance. And our Code tells everyone that we strive to prevent any environmental damage that could result from our work, adding
"We do this by measuring and understanding the environmental impact of what we do, and putting effective systems in place to manage, minimise and prevent it."
These are promises we must keep.
In this case we have agreed to a contract, subject to there being proper measure to protect the local environment. That is a condition of our work. Now the client is trying to avoid the expense.
This is a breach of contract. But it also goes directly against our Code and who we are. So without these measures in place, it would be wrong to continue with the contract, and we would be perfectly entitled to walk away, or even pursue the client for breach of contract.
However, when talking about how we try to continuously improve our environmental performance, our Code also says this:
"We also encourage and help our customers, contractors and suppliers to do the same."
So here we might try to persuade the customer that it is in everyone's interests to pursue a positive outcome, and work with them to try to find alternative sources of funding.
However, we must make it clear that we will not undertake any work that is environmentally irresponsible, and if a solution cannot be found, we must refuse to do the job.