The slurry from biogas plants is an enriched manure because it contains high levels of certain nutrients. Option 1 states that the slurry is rich in phosphorus and potassium. Phosphorus is an essential nutrient for plant growth, playing a crucial role in energy transfer and cell division. Potassium is another essential nutrient that helps in various plant functions such as water regulation and photosynthesis. Therefore, option 1 correctly identifies the nutrients that are abundant in the slurry, making it a valuable fertilizer.

Options 2, 3, and 4 are not correct. Option 2 suggests that the slurry is rich in sulfur and nitrogen. While these nutrients are important for plant growth, they are not mentioned in the question and do not contribute to the enrichment of the slurry. Option 3 suggests that the slurry contains zinc and aluminum, which are not typically found in high concentrations in biogas plant slurry. Option 4 states that the slurry contains boron and potash, but again, these nutrients are not specified in the question or commonly found in biogas plant slurry.

In summary, option 1 correctly identifies phosphorus and potassium as the nutrients that make the slurry from biogas plants an enriched manure.