Is this a geometric sequence: 256, 192, 144, 108.

Yes

Step-by-step explanation:

For GP, Tn = ar^(n-1), where a = first term and r = common ratio, again,

r = Tn/Tn-1

All we will do is to verify

if T2/T1 = T3/T2

 192/256 = 144/192

 0.75       =  0.75

Conclusion:

 As T2/T1 = T3/T2 which is 0.75  =  0.75, therefore, It is a Geometric Progression (GP)