The correct answer is - a. invention of paper.
The Han Dynasty is the Chinese dynasty that is considered to be the greatest, most prosperous, and most influential dynasty of all Chinese history, so much so that the modern Chinese people identify themselves as Han Chinese, despite the dynasty not being in existence for almost two millenniums.
The Han Dynasty had lot of achievements, and one big invention it had was the production of paper. They managed to create the paper that became one of the most important things in the world, used for anything from political agreements, economic deals, art, poetry, writing down the history and what not.
Other inventions of the Han were the iron agricultural tools, the compass, acupuncture, the seismoscope, the wheelbarrow, the rudder...
