This sentence should be: "Le tracce sono chiare." There is a general rule for the feminine plural of all words ending in -cia or -gia. (Like; arancia, camicia, spiaggia, etc)
If there is a vowel in front of the -cia/-gia in the singular form of the word, the plural becomes -cie/-gie:
La camicia --- Le camicie
La ciocia --- Le ciocie
La battigia --- Le battigie
If there is a consonant in front of the -cia/-gia in the singular form of the word, the plural becomes -ce/-ge, without the 'i' !
La arancia --- Le arance
La spiaggia --- Le spiagge
La traccia --- Le tracce
There is (as always) an exception to this rule: When the 'i' in -cia/gia has an accent on it, it should always included in the plural too:
La allergìa --- Le allergìe
La bugìa --- Le bugìe
Lastly, there are some words that are accepted in both forms:
La provincia --- Le provincie / Le province
La ciliegia --- Le ciliegie / Le ciliege
Hey, just a note: allergia and bugia (and farmacia which also falls into this category) do not actually have the accents marked. It's just that the stress falls on the "i".
(Sorry to pick it apart a bit - just trying to understand it!)
I am a native English speaker. The use of the word "traces" in this context is weird. In this context, you would probably say "traces of evidence". "Evidence" (which as noted below is neither singular nor plural, would be a good way to say it in English. I didn't use that word because in the Italian, it is the plural verb, so I knew Duolingo was looking for a plural noun! Note: In English, the word "clues", meaning several pieces of evidence, might be a good fit here.
Evidence? Is this sentence necessarily about some crime?
I was thinking of traces of coffee on the wall.
I put inside the word "evidences" , but it say that isn't correct,Why, I didn't understood, because it is plural of word evidence wich is correct answer.
"The evidence" in English means any and all pieces of evidence be it one item or many. It is an uncountable noun. Similar to knowledge, information, advice, etc.
I know that evidences does not exist, but isn´t "the evidence are clear" correct? as in "the pieces of evidence are clear"
See what has been discussed above - evidence is never used in the plural in English, as it suggests a single collection of pieces of evidence/proof.