5s is present has n=5, l=0 in quantum numbers
n, the principal quantum no, gives us the energy where the electron is located. This is same to the period in which the element is located as well.
l tells us the subshell where the electron can be found. This is equivalent to the blocks in the periodic table where
l= 0 --> s block l=1 --> p block l=2 -->d block l=3 -->f block
So the molecule is in period 5, in the s block which can hold 2 electrons.
So the answer for this one is 2