8) A, B, C, D

Resistors in parallel have a lower overall resistance than same resistors in series.

A has lowest resistance. Now, B only has one resistor in each branch but C has two in the top branch. Hence, C has a lower resistance than B. All resistors in D are in series hence D will have the greatest resistance.

If you are still confused, let us assume that each resistor has a resistance of 2 ohms.

For A,
½ + ½ + ½ = 1/RT
³⁄₂ = 1/RT
R(total) = ²⁄₃ ohms

For B,
Since 2 resistors are branched, we can use the formula,
(R1x R2) = (2 x 2) = 4/4 = 1 ohm
(R1+ R2)    (2 + 2)

For C,

Resistor on top are in series so we can add them up, 2 + 2 = 4 ohms
(R1x R2) = (4 x 2) = 8/6 = 1.33 ohm
(R1+ R2)    (4 + 2)


For D,

we just add everything
2 + 2 + 2 = 6 ohms

The order would be:
A → B → C → D