This will work if the last option is the text entry box. See the demo here.
Qualtrics.SurveyEngine.addOnReady(function () {
text_box = this.getChoiceContainer().querySelector(".TextEntryBox");
text_box.hide();
n_choices = this.getChoices().length;

this.questionclick = function (event, element) {
if (element.type == "radio") {
if (element.id.split("~")[2] == n_choices) {
text_box.show();
} else {
text_box.hide();
}
}
};
});

That's worked wonders. Thank you ahmedA

Hi again! I have a quick question for the same java script - how do I change the code to display the box for multiple choice answers, instead of single choice (radio)?

Here's the revised script that should work with almost all question types, Single select, multiple select, horizontal and vertical:
Qualtrics.SurveyEngine.addOnReady(function () {
    var text_box = this.getChoiceContainer().querySelector(".TextEntryBox");
    text_box.hide();

    this.questionclick = function () {
        var text_choice = text_box.ancestors()_1].childrent1].className.includes("checked");
        if (text_choice) {
            text_box.show();
        } else {
            text_box.hide();
        }
    };
});

https://www.qualtrics.com/community/discussion/comment/33972#Comment_33972Once again, you've been an amazing help. Thank you ahmedA

https://community.qualtrics.com/XMcommunity/discussion/comment/33972#Comment_33972Is there a way to make this code work for a column alignment? I have 17 options I need to include and if I display the responses vertically that will take up too much screen space.