Historical Rules · 1 June 2014
Rule 700.2b
Official source text from the 1 June 2014 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.)