How to write the elements $\mathrm{Se}, \mathrm{Rb} \& \mathrm{S}$ in decreasing atomic size?

The electronegative nature of an element determines its atomic size, therefore if an element is extremely electronegative, its size will decrease because the outermost electrons in its shells will suffer a greater effective nuclear charge from the nucleus.

In the modern periodic table, the atomic size of elements decreases along the period and increases down the group. In the given elements Rubidium is in 5th period, selenium is in the fourth period, and sulfur is in the third period. So Rb will have the highest atomic size, and sulfur will have the least size.
In easier words, atomic size increases with atomic number. Thus Rb with atomic number 37 has the highest atomic size and S having atomic number 16 will have the least atomic size. The correct order is Rb>Se>S.

Hence, option B is correct.