The equivalent circuit of Tunnel diode is shown in the following Figure. The resistive cut off frequency is given by:

concept:

Tunnel diode:

Tunnel diode has very high doped P-N junction. it has negative resistance.

Equivalent circuit diagram of tunnel diode is:

where R is negative resistance of tunnel diode.

R_s is resistance of bulk material and contacting plates.

C is  junction capacitance 

L is parasitic series inductance

To define practical frequency range of a tunnel diode,resistive and reactive cutoff frequency are considered.

Resistive cut off frequency : It is calculated when real part of input impedance is equated to zero.

f_r =   \(\frac{1}{2\pi R\ C_j}\) \(\sqrt{\frac{R}{R_S}\ -\ 1}\)

Reactive cut off frequency : It is calculated when imaginary part of input impedance is equated to zero.

f_x = \(\frac{1}{2\pi}\) \(\sqrt{\frac{1}{LC_j}\ -\frac{1}{{(RC_j)}^2}}\)

so option 2 is correct.