The Optiongroup has an attribute Allow multiple selections. If this is checked (default), the list acts as a group of checkboxes and the Value is a character string with a comma-separated list of the numbers of the checked boxes. If Allow multiple selections is unchecked, the group works as a set of radio buttons and the Valueis a numeric with the number of the single selected button.

You need to get the caption for the selected button:
m.caption = optgrp.buttons(optgrp.value).caption