This is not possible. You can assign users to be able to view the dropbox folder, but thats where it ends. If you really want to create a place inside your dropbox folder that requires a password, then you'll have to use something like a zipfile or truecrypt file. But that is not really the option either because updating the file means it has to sync the entire folder every time.
The only other option for protection is having multiple users with passwords and assign them access to the dropbox folder. A user that doesn't have access won't be able to access it, but if you use only one user, then its just not possible.
possible duplicate of Is there a way I can password protect a dropbox directory on my work pc?, Password protect a folder
– Ƭᴇcʜιᴇ007 – 2014-04-13T21:13:17.5001Why don't you password protect your system? – joeqwerty – 2014-04-13T22:54:04.203
Hi joeqwerty i am using dropbox at my office, so many people may uses my system. So i don't want to set password to pc – kiran lanke – 2014-04-14T05:44:06.887
This is not an answer to your question, but possibly an answer to your problem. Have you considered not using the dropbox client at all, so the data is not synced locally, and instead only use the webinterface? That way you can download the files you need, delete them afterwards and upload changes. In order to access the files, they'd need your dropbox password. – LPChip – 2014-04-14T11:43:52.277