Let the potentials at A and B be VA and VB respectively.
i) If VA > VB
Then current flows from A to B and the diode is in forward biased.
Eq. Resistance = 10/2 = 5Ω.
ii) If VA < VB
Then current flows from B to A and the diode is reverse biased.
Hence Eq.Resistance = 10Ω.