First Principles Thinking: core idea
A is correct. The primitive is total equals count times average. Multiply 45,000 by EUR 22,222 to get EUR 999,990,000, which is essentially EUR 1,000 million. That is the closest choice, so A is correct.
Why the other options are wrong. B is incorrect because it is far below the computed total from the stated loan count and average balance.
Why the other options are wrong. C is incorrect because it is above the computed total and not the closest estimate.