Concept:
The standard of living of people across countries is generally compared using income-based measures that account for differences in purchasing power.
The most commonly used indicators are:
• Per Capita Income (PCI).
• Purchasing Power Parity (PPP).
Step 1: Examine each statement.
• [(A)] Per Capita Income expressed in a common international currency helps compare income levels across countries. Correct.
• [(B)] Purchasing Power Parity adjusts for differences in price levels and is one of the best measures of living standards. Correct.
• [(C)] Head Count Index measures poverty, not the overall standard of living. Incorrect.
• [(D)] Political instability affects development but is not a direct measure of living standards. Incorrect.
Thus,
\[
\boxed{A \text{ and } B}
\]
Hence,
\[
\boxed{\text{Correct Answer}=(B)}
\]