Historical Rules · 27 March 2015
Rule 603.3c
Official source text from the 27 March 2015 Rules release
If a triggered ability is modal, its controller announces the mode choice when he or she puts the ability on the stack. If one of the modes would be illegal (due to an inability to choose legal targets, for example), that mode can’t be chosen. If no mode can be chosen, the ability is removed from the stack. (See rule 700.2.)