○　正解です。

If somebody is happy we can say :
She is like a dog with two tails.

When dogs are happy, they wag (振る）their tails. So a dog with two tails would be twice as happy.

Another simile (直喩), involving dogs, that means to be very happy is :
as happy as a flea (ノミ) in a doghouse.

For example
My son got a new TV game for his birthday and now he is as happy as a flea in a doghouse.