Best way to store a value which could be multiple types

It would appear that you have already been told that you are straying down the road to the EAV model. Take a look at the image here for why the EAV model should be avoided at virtually all costs.

Bill Karwin, the author responsible for the image above, has written a book "SQL Antipatterns: Avoiding the Pitfalls of Database Programming" and he devotes the first chapter to the EAV anti-pattern. He's also a big hitter on this group and huge on StackOverflow (for database issues).

My advice would be to have a table for each type of result and then to use VIEWs to combine them when necessary.

As an example, you could have

CREATE TABLE char_result
(
  question_id INT,
  user_id INT,
  cresult CHAR,
  result_correct BOOLEAN (or equivalent in your RDBMS)
  ..
  <other stuff>
  ..
);

Do the same for num_result, except substitute nresult INT (FLOAT... whatever) for cresult - same idea for VARCHAR &c.

Then create VIEWs over your different result tables for result_correct (and other fields - number_of_attempts... &c. - whatever your other fields are). In this case, you are comparing like with like and not peforming calculations equivalent to adding population to altitude as per the hilarious image referenced above!

I think if it was absolutely necessary to store a value this way in a relational database, I'd use a solution like Microsoft SQL Server's JSON columns.