The "Clear my choice" option to clear radio buttons and checkboxes is currently not available for multiple choice sub-questions, but you can easily add a short JavaScript code that provides it. The script works for any mutiple choice sub-question with radio buttons or checkboxes, i.e.:
MULTICHOICE_V or MCV
MULTICHOICE_H or MCH
MULTIRESPONSE or MR
MULTIRESPONSE_H or MRH
MULTICHOICE_VS or MCVS
MULTICHOICE_HS or MCHS
MULTIRESPONSE_S or MRS
MULTIRESPONSE_HS or MRHS
The script is available in the question text of the following examples. You just have to copy and paste it into your questions. There is nothing else to do.