As @Arun_Patidar mentioned , you have to write custom Jquery to handle these. In you script , you have to see when the option b is selected , using script , set the property "aria-required" true , for these elements. I have done something similar in my proj , where I had to make fields mandatory when the tab is visible. Its the same logic. When your options are visible, you can use the below logic to set the fields mandatory
$("name='./<your field name>'").attr("aria-required", "true");
There are few more things you have to take care
While setting the field true, add * symbol to the label via jquery
When the field is hidden, make sure you set the above values to false and remove the * via jquery
If you are not good with Jquery , get help from a FE developer.