"Be quiet for a minute of silence!"
Translation:Taisez-vous pour une minute de silence !
Generally, announcements in public places in England usually ask people to "respect" or "observe" a minute of silence, though the translation here is obviously more literal.
"sois silencieux pour une minute de silence" is marked wrong. Why? For repetition maybe?
You can be "tranquille" and still speak.
"sois/soyez tranquille" is idiomatic and means "rest assured", "relax" or "be at peace".
To be quiet / to keep quiet / to stop speaking = se taire / être silencieux.