Explanation:

Computers are digital devices, meaning they process and store information in discrete, binary forms. This form of information is known as digital data. Digital data is represented by binary digits (bits), typically $0$s and $1$s. (a) Analog data, in contrast, is continuous and varies smoothly over time (like sound waves or light intensity). While computers can process analog data, it must first be converted into digital data using an Analog-to-Digital Converter (ADC) before storage or processing. (d) Watts data refers to units of power, not a format for storing information. (c) Modem data refers to data transmitted via a modem, which can be analog (over telephone lines) or digital (over cable/fiber), but the data itself, once inside the computer, is digital. Therefore, information on a computer is stored as (b) digital data.