(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