The given sentence "Social behaviour ___ by..." is the passive voice form of the sentence in the passage: "Etiquette is... that governs social behaviour".
Option B: The most suitable option is "is governed", as mentioned in the quoted line: "Etiquette is the unwritten code that governs social behaviour." Hence B is the correct option.
Options A and C: The given sentence is in passive voice, hence the verb must also be in passive form. 'Decides' and 'shows' are active verbs. Moreover, they do not provide the same meaning as 'govern'. Hence A and C are incorrect.
Option D: 'Is shown' does not mean the same as "governs" from the passage. The latter word means 'to control' whereas the former (show) means 'to make visible'. Hence D is also incorrect.