XmToggleButton
Motif's on/off widget, which becomes a checkbox or a radio button depending on a single resource.
State lives in XmNset, and every change is reported through XmNvalueChangedCallback. Appearance is controlled by XmNindicatorType: XmN_OF_MANY gives a square for independent choices, XmONE_OF_MANY an indicator for choices where only one may hold. In Motif 1.x that indicator was a diamond rather than a circle, and those little rhombuses were one of the surest ways to spot a Motif application from across the room.
Mutual exclusion is not the buttons' job but the parent's: an XmRowColumn with XmNradioBehavior, most easily made with XmCreateRadioBox, switches the others off when one is switched on. Motif 2 added a third, indeterminate state for checkboxes that stand for a mixed selection.