The single best verb tense in this situation is the present perfect continuous (have been + verbroot +-ing), which is used to describe actions that began in the past but have continued to the present moment and may not yet be completed.
I have been working on [task name].
or (using a contraction)
I've been working on [task name].
Boss: What did you work on yesterday, and what will you be working on today?
You: I've been working on [task name].
Note: Strictly speaking, if you say here that you "have been working on" a particular task, although it means that you already worked on the task yesterday and today, it does not necessarily mean that you will also continue to work on the same task later today. It just means that you started working on the task in the past, continued working on it until now, and, in this case, you have not yet completed it. (It could mean, as one example, that you will now do something else for the rest of the day and then resume working on the task next week.)
But apart from that very strict interpretation, in the particular scenario described in the original question if you only said "I have been working on [task name]" in response to your boss' question, your boss would reasonably conclude that your plan was to continue working on the same task later today.