i have 3 fields(questions) in a survey:
field 1: name of hospital
field 2: area office
field 3: category.

i want when i select name of hospital(from a drop down list), it should autofill area office and category automatically?

Where should the information for area office and category come from?

How can limesurvey know which information to add depending on the previous choice?

This probably can't be done with limesurvey.

the information is in an excel table, linking area office and category from the hospital name.(in one excel row)and its static(never changes). i can do a label set with dropdown field on hospital name(importing from excel) but what next?

A survey is used to gain data from other people. If you have this data, why do you want to put it into the survey?

let me put it in another way. we have 1200 hospitals in an organisation. the first survey question is from which hospital the respondent comes from, different hospitals fall under different area offices and categories. inorder to avoid data entry errors when carrying out surveys, we just need to select the hospital name, and the area office should show up!the purpose of the excel sheet is just to match the hospital, area office and category.

Hmm, you'll need some kind of workarounds for this. Check manual -> workarounds "Prefilling survey answers using the survey URL"

and manual -> "Setting Conditions"

This won't be easy to implement because limesurvey wasn't designed to feed forms with data which is already available.