iOS9 Untrusted Enterprise Developer with no option to trust

Do it like this:

Go to Settings -> General -> Profiles - tap on your Profile - tap on the Trust button.

but iOS10 has a little change,

Users should go to Settings - General - Device Management - tap on your Profile - tap on Trust button.

Reference: iOS10AdaptationTips

For iOS 9 beta 3,4 users. Since the option to view profiles is not viewable do the following from Xcode.

1. Open Xcode 7.
2. Go to window, devices.
3. Select your device.
4. Delete all of the profiles loaded on the device.
5. Delete the old app on your device.
6. Clean and rebuild the app to your device.

On iOS 9.1+ n iOS 9.2+ go to Settings -> General -> Device Management -> press the Profile -> Press Trust.

In iOS 9.1 and lower, go to Settings - General - Profiles - tap on your Profile - tap on Trust button.

In iOS 9.2+ & iOS 11+ go to: Settings - General - Profiles & Device Management - tap on your Profile - tap on Trust button.

In iOS 10+, go to: Settings - General - Device Management - tap on your Profile - tap on Trust button.