Is there any default option in iPhone 6 to lock the downloaded applications?

Is there any default option in iPhone 6 to lock the downloaded applications?

No, not unless you jailbreak it (which I don't recommend doing).

If you mean so that only a permitted user can add or remove them, then yes. But you'll need to enable the enterprise features on the phone which are often used and set by companies which issue them to their employees.

The day to day user gets a passcode (or may use a fingerprint) to use the phone, but apps can only be added or deleted by an administrator. On my work iPhone 6 the admins have also made sure that employees can't use a lot of the features in Settings, can't update the OS (vital, as our own specialist corporate app needs to be checked and altered to ensure compatibility), and Siri is disabled. Location Services is also permanently turned on so that the company may check where the phones are at any given time (very usedul for lost/stolen phones when the user has forgotten their login credentials for Find My iPhone).