Related Rule
Ukraine
Practice Relating to Rule 21. Target Selection
Ukraine’s IHL Manual (2004) states:
When a choice is possible between several military objectives for obtaining a similar military advantage, the objective to be selected shall be that the attack on which may be expected to cause the least danger to civilian lives and to civilian objects. 
Ukraine, Manual on the Application of IHL Rules, Ministry of Defence, 11 September 2004, § 2.4.2.