◊ 'reasonable'
When someone is reasonable, they behave in a fair and sensible way.
Our mother was always very reasonable.
I can't do that, Morris. Be reasonable.
If something such as a proposal or judgement is reasonable, it is acceptable because it is fair or sensible.
Rules and procedures need to be accepted as reasonable by those who operate them.
There was no reasonable explanation for her decision.
◊ 'rational'
You say that someone is rational when they are able to think clearly and make decisions and judgements based on reason rather than emotion.
Let's talk about this like two rational people.
You can also describe people's behaviour as rational.
This was a totally rational response to a set of complex problems.
Useful english dictionary. 2012.