Hi @Millie,
Just to make sure I follow, you want the respondent to select one (and only one) of the 25 bubble choices in that Matrix grid?
If that's the case, you could set up Custom Validation like the example below. However, it's possible that many respondents will click more than one choice (on a different row from their first choice) and have no way of deselecting their unwanted response.
A better option would be either to add a "Not applicable" column that the respondent would select for the rows that do not apply, or use a drop-down question instead and merge the row and column options together so you have 25 options: "Situational: less than 20%", "Situational: 20-40%", etc.

Hi MathewM. Thank you very much for your answer. I really appreciate it.
I tried the option you described and came to the same conclusion. But I have 8 blocks with each 5 questions and I am trying to find another solution in order to simplify the development of the survey.
Do you have any other idea how to get to a single answer out of a combination of 5 x 5 ? (I tried the solution attached). But I am afraid that I won´t be able to create an index coming out of the answers.

In that case, I just remembered that you can arrange multiple choice single answer questions in columns, so I think this might be your best option:

I would like to know how to enforce a single answer in matrix likert type?Thank you MathewM!!! That could be actually a good solution!