We know the tax can’t be effectively enforced before the end of next year, but we don’t know when the mayor plans to enact the tax. (A) tells us the mayor plans to enact it before the required system is in place, so we know it will initially not be well-enforced.

The argument is unconcerned with the city’s overall financials or whether or not it will incur a deficit. We are only interested in the enforcement of this one specific tax, so (B) is not necessary.

Eliminating (C) relies on understanding the distinction between is and ought. Just because it should be implemented as soon as it can be enforced gives no indication of when it will actually be implemented.

The city’s rationale behind the new tax is purely contextual and irrelevant to the argument. Regardless of what the city cares about more, it is still entirely possible that the tax is implemented before the system is in place, and thus initially poorly enforced.

Whether or not this tax succeeds in reducing congestion is irrelevant to the argument, and it certainly doesn’t have to be the best way to do it. Other more effective methods could exist, with no impact on whether or not this tax will initially be well-enforced.