Translation:The firefighters have the hectoliter of water.
When i did this exercise and came to report it it was a listen and type in Romanian exercise and the lady said hectolitrul so much as the English literal translation would never be heard spoken that is the answer.
I am going to assume their fire engine (tender) contains a hectolitre of water and that is "the" hectolitre referred to in the sentence.
In the "insert missing word" version of this question, there's not enough grammatical information to indicate the correct answer.
I am of the same opinion ...
the romanian sentence is not long enough to offer enough information in its context ...
so , both answers are possible as missing words .... !
probably they had in their mind a sentence like :
"pompierii au ...... de apa ca unitate de masura" / "the firefighters have ..... of water as unit of measurement " ....
in this case the singular is ok ...
Unfortunately it is not possible to report this variant as “my answer should have been accepted.”
Heck, I don't know what a hectolitre (hectoliter in American) of water is and doubt many English firemen could put me right without looking it up on the web or in a dictionary!