The equivalent resistance between A and B as shown in figure is:

$$\text{Node } A \text{ is directly shorted to Node } D \implies V_A = V_D$$

$$\text{Node } B \text{ is directly shorted to Node } C \implies V_B = V_C$$

$$\text{First } 20\text{ k}\Omega\text{ resistor is connected between nodes } A \text{ and } C \implies \text{connected between } A \text{ and } B$$

$$\text{Second } 20\text{ k}\Omega\text{ resistor is connected between nodes } C \text{ and } D \implies \text{connected between } B \text{ and } A$$

$$\text{Third } 10\text{ k}\Omega\text{ resistor is connected between nodes } D \text{ and } B \implies \text{connected between } A \text{ and } B$$

$$\text{All three resistors are in parallel across terminals } A \text{ and } B: \frac{1}{R_{AB}} = \frac{1}{20} + \frac{1}{20} + \frac{1}{10}$$

$$\frac{1}{R_{AB}} = \frac{1}{10} + \frac{1}{10} = \frac{2}{10} = \frac{1}{5} \implies R_{AB} = 5\text{ k}\Omega$$