Most important layer of a slow sand filter is -
This is confusing one because of the following statements of Park.
	"The most important part of the filter is the sand bed" - Park
	"The vital layer is the heart of the slow sand filter" - Park
Following statement may help in choosing the answer : -

"Vital layer is the layer that provides the effective purification in potable water treatment, the underlying sand layer providing the support medium for this biological treatment layer"  - Water & Health 3rd/e 733.

Read the mechanism of action of slow sand filter and there will be no confusion.
Mechanism of action of slow sand filter
Slow sand filter work through the, formation of a gelatinous layer called vital layer or hypogeal layer or Schmutzdecke in the top few cm of sand layer.
This layer is formed in the first 10-20 days of operation.
The formation of vital layer is known as "Ripening of the filter".
This vital layer consists of bacteria, fungi, Protozoa, and a range of aquatic insect larvae.
As waste passes through vital layer, particles of foreign matter are trapped in the mucilaginous matrix and dissolved organic material is absorbed and metabolized by bacterial, fungi and protozoa → Vital layer has mechanical (physical) as well as biological action.
So, the vital layer acts as the heart of slow sand filter.
Vital layer
Sand bed
Filter system
Raw water