I don't see a problem with A. And B is correct too. It's just that the focus is different. A focuses on an ongoing action very close to the moment of speech, whereas B includes a longer span of time and can be interpreted as a simple future.

I imagine A being said in an environment where a task is ***being carried out***, and ***immediate*** assistance is offered in case of need.

I understand B said in an identical situation as A, but it is not limited to it. It is also possible when someone carries out a task in an unspecified time and until that task comes to an end, assistance is offered in the future.

[This site][1] states:
> In the condition clause we can use present continuous:

> * If I’m driving, I don’t talk on my phone.

A [language blog][2] says:

> *Imperatives with the zero conditional*
> 
> We often use the zero conditional ***to tell people what to do, or to give instructions***. To do this, we use the structure: ***If / When + present tense, imperative***
> *  When the water ***is boiling***, **add** the pasta. 


  [1]: https://In%20the%20condition%20clause%20we%20can%20use%20present%20continuous:%20%20If%20I%E2%80%99m%20driving,%20I%20don%E2%80%99t%20talk%20on%20my%20phone.
  [2]: https://www.eslbase.com/grammar/zero-conditional#:~:text=Using%20present%20continuous%20and%20modal%20verbs%20in%20zero%20conditional&text=In%20the%20condition%20clause%20we,working%2C%20my%20boss%20is%20happy.