(a) For obtaining the highest resistance by combining the given resistances, we must connect them in series.

We get,

R = 4 + 8 + 12 + 24 = 48 ohms

(b) For obtaining the lowerst resistance by combining the given resistances, we must connect them in parallel.

We get,

1/R = 1/4 + 1/8 + 1/12 + 1/24

On solving we get, R = 2 ohms