Historical Rules · 16 July 2010
Rule 700.2b
Official source text from the 16 July 2010 Rules release
The controller of a modal triggered ability chooses the mode(s) as part of putting that 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 603.3c.)