Regular and Irregular Plurals
Most of the time, we simply add an "s" to a noun to make it plural (if it doesn't have one already, or if it ends with an "x" or "z").
When the noun ends in "'-al", the plural ending is "-aux", e.g. un animal - des animaux; un cheval - des chevaux, un bocal - des bocaux, etc. (These are all masculine nouns.)
(Note: there are a few exceptions to this rule, e.g. un festival - des festivals, NOT: des festivaux.)
Consequently, the same rule applies to adjectives ending in "-al" when modifying masculine, plural nouns, e.g. un animal tropical - des animaux tropicaux.
Generally, if a noun ends in "-eu", "-eau" or "-au", it takes an "x" in the plural, e.g. un feu - des feux, un agneau - des agneaux, etc.
Also there are precisely 7 nouns ending in "-ou" which take an "x" rather than an "s" in the plural: bijou, caillou, chou, genou, hibou, joujou, pou.
Just as a funny note, don't think these things are all easy for native French speakers either -- lots of us have trouble with some « aux » or « eaux » plural nouns (especially since there are exceptions, of course : the plural for the masculine adjective « naval » is... « navals » indeed, whereas it supposedly "should be" « navaux » !).
And my guess is every French people remembers having to painfully learn that dreadful « oux » plural nouns in school, although I'm pretty sure lots of French grown-ups wouldn't be able to recite it really easily, or in fact at all (even if they might instinctively write these plural nouns correctly) :)
That's the 'problem' with every language I've encountered so far! -- your native language doesn't have its own abundance of exceptions? Though I'd say it makes learning a language more interesting ( and getting those exceptions right would surely get you some points from native speakers ;D ).
Also remember when doing the plural section, EVERYTHING is going to be plural or have a plural part of the sentence, you should rarely be using le, la, l', un, or une. Remember not to use the l' even with something like elephants. Des and les are going to be common and should be assumed on all nouns except things like viande, and l'eau.
But don't just assume 100% of everything is plural either they have a couple sentences where they say something like: a gender noun and some othergender nouns do action, to directly compare the difference. Every sentence is going to have a plural in it, in some way or form, if your answer doesn't have a plural in it it will most likely be wrong.
If you are just barely getting into french (Less than a week in) and have been able to fly through to this point, this is going to be the hardest lesson for you to learn yet. Just keep practicing everyday even if you can't get past this lesson and whatever you do not try to memorize the answers or look it up on google translate. Well memorizing the answers would probably help more than google translate so do that before you cop out to google.
you may find useful a picture to summarise the irregular plural in French: http://www.frenchspanishonline.com/magazine/french-irregular-plurals/