The most appropriate meaning of the idiom "to be thrown in at the deep end" is learn to do the difficult things too soon.

This idiom means to be put into a new and difficult situation without any preparation or introduction. It is often used in the context of starting a new job or project.

For example, a new employee who is asked to give a presentation to the CEO on their first day of work could be said to be thrown in at the deep end.

The other options are not as idiomatic or do not accurately convey the meaning of the idiom.

• Need to do one thing at a time - This is not a meaning of the idiom "to be thrown in at the deep end."
• Try to do too many things at the same time - This is not a meaning of the idiom "to be thrown in at the deep end."
• Unable to do the given work - This is not a meaning of the idiom "to be thrown in at the deep end."