Answer:
The right answer is nouveaux.
Step-by-step explanation:
There are two main things to consider when agreeing an adjective with a noun:
1) the gender of the noun
2) whether the noun is plural or singular
In this case:
1) the noun "chiens" is masculine. You can also tell it is masculine because of the way that the adjective "petit" has been agreed without an "e".
2) The noun is plural, as there is an "s" at the end of "chiens".
Since the noun is masculine, you can rule out "nouvelle" and "nouvel" straight away. You now have to choose between "nouveau" and "nouveaux". Since the noun is plural (there is more than 1 dog), you would choose "nouveaux".
Hope this helped : )