Concept:
Assets recorded in accounting books may lose value because of damage, obsolescence, market decline, or reduced usefulness.
Step 1: Understand the term “book value”.
Book value is the value at which an asset appears in accounting records.
Step 2: Understand the meaning of write-down.
When the actual value of an asset becomes lower than its recorded value, accountants reduce its value in the books.
This reduction is known as a write-down.
Step 3: Differentiate between write-down and appreciation.
A write-down decreases asset value.
It is not an increase in value.
Hence Option (A) is incorrect.
Step 4: Select the correct definition.
A write-down means:
\[
\text{Partial reduction in the book value of an asset}
\]
Therefore,
\[
\boxed{\text{Option (B)}}
\]
is correct.