How to correctly name a single-select element in forms?

How to correctly name this in English (in an official documentation for an IT system or in an instruction for filling up some form):

enter image description here

Should I name this:

Solution 1:

Similar question resulted with an answer that form with space is much more popular and often used.

Simiral Google Ngram search reveals that:

writing with space ("radio button" and "option button") are much more popular,
"optionbutton" and "option-button" was never used or is not found withing Google Ngram,
"radiobutton" and "radio-button" usage is very low.

It seems that (similarly to "check box"), "radio button" is the most widely used.

How to correctly name this in English (in an official documentation for an IT system or in an instruction for filling up some form)

These are two different requirements:

In your official documentation, you should use the name that your application uses for this element. For example, if your solution is written in HTML, then you should use the term radio buttons:

Radio buttons are like checkboxes except that when several share the same control name, they are mutually exclusive: …
For user instructions, use whatever is clear to your users. In most cases, you don't need to name the element as such at all - you can say simply:

Select • under age if you are less than 18 years old;

If you do need to refer to it, I believe option would be the best non-technical term to use:

Select the • under age option if you are less than 18 years old;