Historical Rules · 1 October 2010
Rule 603.3c
Official source text from the 1 October 2010 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.)