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?
Repetition is not a grammar error.
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.
What's wromg with "un moment" instead of "une minute"?
I answered in the plural and got it wrong!