Reasonable means that whatever decision was made is appropriate given the particular circumstances, and most would regard it as so.
Justified means that whatever decision was made is done for a good reason, in the interest of fairness.
There are times the words could be used interchangeably, and contexts where the difference is subtle. It all depends on the nature of the request or issue at hand.
I might use "justified" if I was angry about something, and was acting in response:
Was it polite to skip their wedding after they insulted me like that? No, but I think my decision was justified.
I might use "reasonable" if I think everyone would agree that there was nothing else that could be done:
Was I happy about missing their wedding so that I could go to my mother's funeral? No, but I think my decision was reasonable.
But there are contexts where either word will work just fine:
You said that you'd deliver the cake before noon, but it didn't get here until 3 o'clock. I think my request for a refund is reasonable/justified.
In that sentence, reasonable would work because most people would agree that a refund would be appropriate given the circumstances, and justified would work because the request is fair.
The only verb that means "working in a certain way" is operate; the other verbs have different meanings.
For example, implement means "to make something that has been officially decided start to happen or be used."
A new work programme for young people will be implemented.
The regulations implement a 1954 treaty.
When speaking of code, I could say "I will implement the new program features over the next week."
As phrasal verb, carry out means "to do something that you have said you will do or have been asked to do" or "to do and complete a task."
Best Answer
It depends on the context in which both words are used. Generally, there is little to no difference. However, a person may use certain to add more power to his statement when he is completely positive. For example:
A greater emphasis is added to one's sentence when using the word certain. On the other hand, when using sure it does not always sound confident, and you can tell that in the way it is pronounced most of the time. For example:
or
With certain you completely believe that you left your mobile at home. I hope this has answered your question.