We must calculate the number of molecules in each case.
a) 1.8 g of water
#"Molecules of water"#
#= 1.8 color(red)(cancel(color(black)("g water"))) × (1 color(red)(cancel(color(black)("mol water"))))/(18.02 color(red)(cancel(color(black)("g water")))) × (6.022 × 10^23 color(white)(l)"molecules water")/(1 color(red)(cancel(color(black)("mol water"))))#
#= 6.02 × 10^22color(white)(l) "molecules water"#
b) 18 mol of water
#"Molecules of water"#
#= 18 color(red)(cancel(color(black)("mol water"))) × (6.022 × 10^23 color(white)(l)"molecules water")/(1 color(red)(cancel(color(black)("mol water"))))#
#= 1.08 × 10^25color(white)(l) "molecules water"#
c) 18 g of water
#"Molecules of water"#
#= 18 color(red)(cancel(color(black)("g water"))) × (1 color(red)(cancel(color(black)("mol water"))))/(18.02 color(red)(cancel(color(black)("g water")))) × (6.022 × 10^23 color(white)(l)"molecules water")/(1 color(red)(cancel(color(black)("mol water"))))#
#= 6.02 × 10^23color(white)(l) "molecules water"#
d) 18 molecules water
We have 18 molecules of water.
There are the most water molecules in Option b).
