Answer
See the detailed answer below.
Work Step by Step
$$\color{blue}{\bf [a]}$$
To have a net resistance of 4 $\Omega$ from the three 12-$\Omega$ resistors, we need to connect the three in parallel as one combination.
See the first figure below.
$$\color{blue}{\bf [b]}$$
To have a net resistance of 8 $\Omega$ from the three 12-$\Omega$ resistors, we need to connect two of them in series as one combination. And then connecting this combination in parallel to the third resistor.
See the second figure below.
$$\color{blue}{\bf [c]}$$
To have a net resistance of 18 $\Omega$ from the three 12-$\Omega$ resistors, we need to connect two of them in parallel as one combination. And then connecting this combination in series to the third resistor.
See the third figure below.
$$\color{blue}{\bf [d]}$$
To have a net resistance of 36 $\Omega$ from the three 12-$\Omega$ resistors, we need to connect the three of them in series as one combination.
See the fourth figure below.
