Step 1: Recall the products of a biogas plant.
A biogas plant produces:
- Biogas (mainly methane)
- Digested slurry
Step 2: Identify the use of slurry.
The digested slurry is rich in nutrients such as nitrogen, phosphorus and potassium, making it an excellent organic fertilizer.
Hence,
\[
\boxed{\text{Fertilizer}}
\]
is the correct answer.
Therefore,
\[
\boxed{(B)}
\]
is the correct answer.