Answer: Third option is correct.
Step-by-step explanation:
Since we have given that
there are two similar televisions at a local electronics store,
Dimension of generic television set is 
12 inches by 24 inches 
We need to calculate the dimensions of brand name television ,
Since it is also given that the generic television was two third of the size of the brand name, i.e.

Now,

So, length of brand name television becomes 

Breadth of brand name television becomes 

So, Dimension of brand name television is 
18 inches by 36 inches 
So, Third option is correct.