Answer:
Country B
Step-by-step explanation:
To determine which country has a higher standard of living, we have to find the per capita GDP.
Per Capita GDP = real GDP/ population 
Real GDP = nominal GDP / GDP deflator 
Country A's real GDP = $600 billion/ 150 = $4 billion 
Country B's real GDP = $720 billion / 120 = $6 billion
For country A, per capita GDP = $4 billion / 40 million = $100 million. 
For country B, per capita GDP = $6 billion / 50 million = $120 million. 
Country B has a higher standard of living because it's per capita GDP is higher than country A.
I hope my answer helps you.