As per Duo rule it must be 'एक गाय...'
"होती है" is accepted as an answer, albeit it is not necessarily correct usage since not all cows are black. Hindi is a rhythmic language and thus the colloquial versions become the norm.
Some cows may have white, red, maroon..
I answered without होती and it was considered correct.
I wrote एक गाय काली हैं। What is wrong (except for the ं on है)?
2 answers in Hindi are the same for this sentence ?
When I clicked on one of them, the system said that it's not a correct answer...