"That is your duty."
Translation:C'est votre devoir.
Le devoir or l'obligation refer to a moral duty. La responsabilité refers to a job or function. All may be translated as "duty" with their respective meanings. http://www.wordreference.com/enfr/duty Usually, when referring to a job, one refers to "duties", not "duty". Using "duty" is generally going to be interpreted in the sense of a moral obligation. Duty could also refer to a specific aspect of one's job, however.
The English "duty" generally includes the idea of an obligation or requirement, or a moral responsibility as part of the job. A duty may be one of the requirements/functions/responsibilities of a job but is usually not the job itself. One can "report for duty" and one's duties (pl) are all the assignments or requirements of the job. It is possible that one may say "your duty is to clean those dishes" which could include job/assignment/chore/task. In that sense, la tâche is accepted.
I feel that merely because the sentence contains the demonstrative pronoun that it is stressed as is. If that was not intended, then "C'est ton / votre devoir" would just be "It is your duty" in English.
In my opinion, a phrase such as "That is your duty" is much better conveyed by:
- Ça, c'est ton / votre devoir. or
- Cela est ton / votre devoir.
Why is it C'est which translated would be "it is". Rather it should "cela est"... "that is". ( as opposed to "ceci" .... this is) I wrote cela est and the duo translators in their wisdom said.... (surprise surprise)...Non! Porquoi? How would you translate cela est votre devoir?
Not really, you were right the first time. It is indeed common to say "C'est de votre devoir de... / Il est de votre devoir de... + infinitive" in the sense of It's your duty / responsibility / obligation to..., It's part of your job to..., You are obliged to....
As for "What is your job (=duty)?" that would be "Quel est votre devoir ?"
Fun fact: there's a newspaper in Quebec called "Le Devoir". It's considered conservative in the Quebec context (separatism, cultural protectionism, and to throw a wrench in the normal narrative, very socialist), but is also well respected and is the only truly independent newspaper in Quebec and one of the only ones in Canada. In the past, its motto was "do what you must" but currently it is "free to think" (libre de penser).