Exports and imports, a swelling favourable balance of trade, investments and bank-balances, are not an index or a balance sheet of national prosperity. Till the beginning of the Second World War, English exports were noticeably greater than what they are today. And yet England has greater national prosperity today than it ever had. Because the income of average Englishmen, working as field and factory labourers, clerks, policemen, petty shopkeepers and shop assistants, domestic workers and other low-paid workers, has gone up.

The passage best supports the statement that:

The passage discusses how national prosperity should not be measured by exports, imports, or balance of trade alone. It points out that England, despite having lower exports than before, has seen greater national prosperity due to the increase in the income of average Englishmen. Here's the analysis of the options based on the passage:

1. Option A - a country's economic standard can be best adjudged by per capita income:
   • The passage highlights that national prosperity is better indicated by the income of the average worker, rather than by trade balance or exports. This aligns well with the idea of using per capita income as a measure of a country's economic standard.
   • This option is strongly supported by the passage.
2. Option B - a country's balance of trade is the main criteria of determining its economic prosperity:
   • The passage explicitly states that exports, imports, and balance of trade are not good indicators of national prosperity, which contradicts this option.
   • This option is not supported by the passage.
3. Option C - a nation's economy strengthens with the increase in exports:
   • The passage discusses that England's exports were higher before the Second World War, but it attributes England?s increased national prosperity to the rise in the income of average workers, not to increased exports.
   • This option is not supported by the passage.
4. Option D - English trade has continually increased since the Second World War:
   • The passage mentions that English exports were greater before the Second World War, and it does not state that trade has continually increased since. In fact, it implies that exports have not increased, yet national prosperity has grown due to other factors.
   • This option is not supported by the passage.
5. Final Answer:
   • The best supported statement by the passage is A. a country's economic standard can be best adjudged by per capita income.