If someone answers a question clearly, you wouldn't really use any of those words. You would say "You answer was clear" or "I understand that now" or something similar. As for the words you ask about, I'd say the following:
"Comprehensible". This is rarely used. However "Incomprehensible" is commonly used. If, for example an essay was so unclear and garbled that it was impossible to understand, you'd say it was incomprehensible.
"Comprehensive". This means "complete". So, if an answer was very thorough and included all relevant information, you'd say it was a comprehensive answer.
"Understandable". You would not normally say, "That text/answer/essay was understandable". This word usually refers to a situation or a state. For example, you might say "Mr A hasn't had any training, so his mistake was understandable". Or, if someone said to you, "Mrs J just lost her job, so she doesn't feel like talking much" you might reply "Oh, that's understandable".
Finally "Realise". This normally has two senses. First, if you realise something, it means something has become clear to you, usually as a result of an experience and over a period of time e.g. "When the day became dark with clouds, I realised it was going to rain".
"Realise" also has a different sense, which is "to make something concrete" e.g. "His ideals were finally realized". It's in the sense that you may use the form "realisable" e.g. "Is this project realisable.
These are all difficult words, but I hope you found my explanations useful