Which of the following is the outermost layer of an underground cable?

The outermost layer of an underground cable is serving.

Construction of the underground cable

1.) Cores or Conductors: 

• A cable may have one or more than one core (conductor) depending upon the type of service for which it is intended.
• The conductors are made of tinned copper or aluminum and are usually stranded to provide flexibility to the cable.

2.) Insulation

• Each core or conductor is provided with a suitable thickness of insulation, the thickness of the layer depending upon the voltage to be withstood by the cable.
• The commonly used materials for insulation are impregnated paper, varnished cambric, or rubber mineral compounds.

3.) Metallic sheath

To protect the cable from moisture, gases, or other damaging liquids (acids or alkalies) in the soil and atmosphere, a metallic sheath of lead or aluminum is provided over the insulation.

4.) Bedding

Over the metallic sheath is applied a layer of bedding which consists of a fibrous material like jute or hessian tape. The purpose of bedding is to protect the metallic sheath against corrosion and mechanical injury due to armoring.

5.)  Armouring

Over the bedding, armoring is provided which consists of one or two layers of galvanized steel wire or steel tape. Its purpose is to protect the cable from mechanical injury while laying it and during handling. 

6.) Serving

To protect armoring from atmospheric conditions, a layer of fibrous material (like jute) similar to bedding is provided over the armoring. This is known as serving.