yakusoku/yotei/yoyaku.

病院に電話をかけて（　　）を変更してもらわなければ。

1.)約束 2.)嵐� 3.)蘭�

Which is the suitable word, and also can any one tell me the differences in the usage of these words?.

3)蘭� is the right one for this sentence.

1) "yakusoku" is a promise - something you make with your friend, your colleagues, buddies, etc.
2) "yotei" is a general schedule.
3) "yoyaku" is an appointment (you make with a doctor, hospital) or a booking (with a hotel, restaurant, etc.).

Thank you AK-san.
I have one more question.

ごめんね。日曜日行けなくて。急にトロントから友達が来ることになっちゃって。

I don't understand the なっちゃって. What does it mean and what type of form it is?

来ることに なっちゃって is the colloquial version of 来ることに　なってしまって, which is the "...te-form + shimau/shimaimasu,"expressing the sense of something happening unexpectedly, and the speaker is feeling "unfortunately..."

So the sentence is something like, "I'm sorry that I cannot come on Sunday. Unexpectedly, suddenly it turns out a friend of mine is going to come from Toronto."

Thanks a lot Ak-san i can able to understand clearly....

In the sentence it is 日曜日行けなくて。but in your translation you have said as "....cannot COME on sunday". Why the verb iku(to go) translated as COME.

日曜日行けなくて。

Sorry, I should have said "cannot go on Sunday."

In my mind I was imagining a situation that the person who said the sentence had maybe been invited to the other person's house or a party or something like that... in that case, in English, you would naturally say "cannot come (to you)." But because I don't know of the specific situation behind this sentence, it should be "I cannot GO on Sunday."