How can I add points for selected answers to questions implemented using Polls?
By using the Rules module, and some extra work, you can get this to work, as detailed below ...

Step 1: Use Rules to grant User Points

Use Rules to create a custom rule with a Rules Action that grants points (using the User Points module). Have a look at the rules exammple included in my answer to "How to grant userpoints only for modifying nodes last changed at least 20 days ago?", especially the very first Rules Action in it. Here is how that Rule looks like:

{ "rules_grant_userpoints_after_updating_old_content" : {
    "LABEL" : "Grant userpoints after updating old content",
    "PLUGIN" : "reaction rule",
    "OWNER" : "rules",
    "REQUIRES" : [ "rules", "userpoints_rules" ],
    "ON" : { "node_update" : [] },
    "IF" : [
      { "data_is" : {
          "data" : [ "node-unchanged:changed" ],
          "op" : "\u003C",
          "value" : "-20 days"
        }
      }
    ],
    "DO" : [
      { "userpoints_action_grant_points" : {
          "user" : [ "site:current-user" ],
          "points" : "123",
          "tid" : "0",
          "entity" : [ "" ],
          "description" : "Modify existing node (nid=[node:nid]) that was not updated in the last 20 days.",
          "operation" : "Grant points",
          "display" : "1",
          "moderate" : "default"
        }
      },
      { "userpoints_rules_get_current_points" : {
          "USING" : { "user" : [ "site:current-user" ], "tid" : "all" },
          "PROVIDE" : { "loaded_points" : { "total_points" : "Number of points in all categories together" } }
        }
      },
      { "drupal_message" : { "message" : "You now have [total-points:value] points" } }
    ]
  }
}


The 2nd and 3rd Rules Action is rather to facilitate testing (you may want to ignore those Actions after QA-testing).

Step 2: Make (core) Poll trigger the Rule

To continue with the Rule from Step 1, you don't want to use that Rules Condition there, so just delete that. However the crucial part for your case is to have an appropriate Rules Event to replace the event used in the Rule from Step 1. In your case you want to use a Rules Event like "When user casts a vote".

Guess what, there seems to be a (sandbox) module that provides exactly that Rules Event: the Poll Rules Integration. If you don't want to use a sandbox project, then just look at this project to see what's needed for that event to build your own custom module to create such event.

Step 3: Further complete your Poll-puzzle

After you complete Step 1 and 2 above, the only thing left to complete your "puzzle", is to implement an appropriate Rules Condition to check "if they answer the test correctly" (as in your question, for which you did not specify any further details).