Answer:
FQI.
Step-by-step explanation:
In the following code the CAT is ECV means that "C" code is represented by "E" , "A " is represented by "C" and "T" is represented by "V" means that every character is replaced by the two places in the alphabet
For example :if the code "B" then next 2 character is "D" means "B" is replaced by "D".
Similarly "D" is represented by "F" ,"O" is represented by "Q" and "G" is represented by "I" so the code of DOG is FQI.