The true statement about money from the options given is:
a. Money maintains purchasing power over time.
However, it's important to clarify that while money aims to maintain purchasing power, it can be affected by inflation, changes in the economy, and other factors, which can lead to a decrease in purchasing power over time. The other statements are false because money does not permanently retain the same value (b), does not stay at the same value over time (c), and does provide a standard unit of measurement (d).