Allow a dropdown custom field to be edit from a front end form
Just to make sure I am on the same page, you have a select field within the control panel that is populated with the prefix/title(s)?

If that's the case, within your registration form, you could do something like this. Not tested, but should be close...

{% set options = craft.fields.getFieldByHandle('yourFieldHandle').getFieldType.options %}

{% if options|length %}
    &lt;label for="prefix"&gt;Prefix&lt;/label&gt;
    &lt;select id="prefix" name="fields[prefix]"&gt;
        {% for option in options %}
            &lt;option value="{{ option.value }}"&gt;{{ option.label }}&lt;/option&gt;
        {% endfor %}
    &lt;/select&gt;
{% endif %}


EDIT

Good catch by @Benjammin'. You will need the options part to actually access the values. I've updated my answer, give it another try.