Which of the following devices is used in voltage regulators and voltage limiters as a fixed reference voltage in the network?

Zener Diode:

The Zener diode operates just like the normal diode when in the forward-bias mode, and has a turn-on voltage of between 0.3 and 0.7 V. However when connected in the reverse mode, which is usual in most of its applications, a small leakage current may flow and the voltage across it is kept constant.

V - I characteristic of Zener diode

Zener as a Voltage Limiter:

A resistor, R₁ is connected in series with the Zener diode to limit the amount of current flowing through the diode and the input voltage V_in (Which must be greater than the Zener voltage) is connected across as shown in the image, and the output voltage V_out is taken across the Zener diode with V_out = V_z (Zener Voltage).

They are used as Voltage limiter and voltage regulator.

Varactor Diode is a type of diode whose internal capacitance varies with respect to the reverse voltage.
It is used in frequency modulation for FM transmitters and phase-locked loops.

Tunnel diode is a type of semiconductor diode that has effectively "negative resistance" due to the quantum mechanical effect called tunneling. They areused in oscillator circuits, and in FM receivers.

PIN diode is a diode where an intrinsic layer of high resistivity is sandwiched between the P and N-region of semiconductor material. They are used in attenuators, fast switches, photodetectors