The answer is the first choice. If every 2 cm. is 4 cm., that means that the dimensions' values are doubled. Then, you can just double the dimensions. Preferably, in my opinion, this is the easier way to go. By your preference, though, you can do it the longer way as well.
There are 4 2s in the number 8.
There are 3 2s in the number 6.
If each of the 2s are 4s,
So, therefore,
Transforming the 2s to 4s,
4x4=16 cm.
3x4=12 cm.
So, this means that the final answer is the first choice. I hoped that this helped answer your question. Enjoy your day, and take care!