Step-by-step Solution:

The correct answer is: \[ \textbf{A. Polarisation} \]
Explanation:
The latest write-once optical storage medium , called digital paper, utilizes the phenomenon of polarisation. Digital paper can change its optical properties (such as colour or light transmission) when exposed to light, and the information is "written" by altering the state of polarisation of the light that reflects off it. This allows the data to be stored in a way that cannot be easily erased, making it a write-once medium.