Explanation:

In computer architecture, a "word" refers to the natural unit of data that a particular processor design handles at one time. The size of a computer's word is fundamentally defined and measured in bits (e.g., a $32$-bit word or a $64$-bit word). While a word can also be expressed in bytes (e.g., a $64$-bit word is $8$ bytes), bits are the most granular and fundamental unit for specifying its length. Option (d) Bits is therefore the most precise answer. Option (a) Byte is a valid unit but less fundamental than bits for defining a word's inherent size. Options (b) Millimeter and (c) meter are units of physical length, irrelevant to data size.