The ability of a conductors insulation to transmit an electric field is called:

Conductivity describes a material's ability to carry electric current, which is the opposite property of insulation - it does not describe how insulation transmits an electric field.

Transmitability is not a recognized term in electrical engineering or telecommunications standards and does not correspond to any defined material property.

Permittivity (also called dielectric constant or relative permittivity) is the precise measure of an insulating material's ability to permit an electric field to propagate through it. In cable construction, the permittivity of the conductor's insulation directly governs electrical characteristics such as propagation delay and mutual capacitance. It is the accepted technical term in electrical and cabling standards for this specific dielectric property.

Capacitance is the ability of a system to store electrical charge and is an effect influenced by permittivity, but it is not the property that defines how insulation transmits an electric field.

Reactance is the opposition to alternating current due to inductance or capacitance in a circuit, not a material property describing electric field transmission through insulation.