The best answer is A. strong because it best describes an economy that has weathered a crisis and remained in a positive, resilient state. The other options do not capture this meaning as effectively.

"Realistic" suggests the economy is grounded in facts, but does not necessarily indicate strength or resilience.
"Tough" implies the economy has faced difficulties, but does not convey that it has remained stable.
"Firm" suggests the economy has maintained a steady, unwavering state, which is similar to "strong" but not as concise and powerful a descriptor.

Therefore, the correct answer is A. strong.