Answer
The order of decreasing radius is:
$Rb \gt Ca \gt Si \gt S \gt F$
Work Step by Step
We know, atomic radius increases down a column of the periodic table and decreases across a row of the periodic table. $Rb$ is in the first row and the fifth column of all the given elements, so it has the greatest atomic radius.
After that, $Ca$ is in the 2nd row and 4th column, so its atomic radius is the next largest.
Then, $Si $ and $S$ are both in the 3rd column, but $S$ is in the 16th row and $Si$ is in the 14th, so the radius of $Si$ is greater.
Finally, $F$ is in the 2nd column and has the smallest atomic radius among these elements. So, the order of decreasing radius is:
$Rb \gt Ca \gt Si \gt S \gt F$
