Enthuware

Option 3.<%! String uid = LoginHelper.login(request) %>
! is used for declarations.//explanation of why it is wrong

Is it wrong because of "String uid = LoginHelper.login(request)" being in declaration, or because of missing ";"?

If you think of it as a declaration then it is wrong for 2 reasons: request parameter is not available in a declaration and missing ;.
If you think of it as a scriptlet, it is wrong because ! is not used in a scriptlet and missing ;.

HTH,
Paul.