The SFDC field "Description" is mapped to Marketo "person notes" which you can also use. Of course, it has the same overwrite issue Cathal notes...which can be troublesome if your sales team is used to writing things in there. Get them to use Tasks!
Many people will create a custom text field in SFDC allow it to sync to Marketo and then add the field to the Marketo form.
It's straightforward if you just need to use a free text field in one form, but if you intend to use it in multiple forms to collect data from different questions then you may have to create multiple fields as is you recycle the same free text field over in many forms the most recent form fill will overwrite the data from the previous one.
.png)
Adchoices