The question asks about where the electric strength in a cable is more prevalent. To answer this, we need to understand how electric fields behave within the structure of a cable.

Electric strength refers to the maximum electric field that a material can withstand under an applied voltage without breaking down. In the context of a cable, electric fields originate from the core conductor and radiate outwards through the layers of insulation.

Let's analyze the options provided:

1. Layer on the conductor: This is the layer of insulation that is directly in contact with the conductor. According to the physics of electric fields, the electric field's intensity is strongest closest to its source, which in this case is the conductor itself. Thus, the electric strength is highest on the layer adjoining the conductor.
2. Outside conductor: While this might seem plausible, it does not account for the relative distance compared to the layer directly on the conductor. The electric strength diminishes with distance from the conductor.
3. Outer side of armouring: The armouring is typically the last layer and is mostly used for mechanical protection rather than insulating purposes. The electric field by this point is much weaker.
4. None of these: This is incorrect as the electric strength actually is greatest at the layer on the conductor.

Therefore, the correct answer is clearly "layer on the conductor". This is because the electric field is strongest where it is closest to the source, and in this context, that is the immediate insulating layer adjacent to the conductor.