Let B $=\{\{1,2\},3\}$

Which one of the following sets is a proper subset of $($B$)?$

$($Remember to distinguish between elements and subsets of $($B$).)$

Select one:

a$.$

$\{\{\{1,2\},3\}\}$

b$.$

$\{\{\},\{\{1,2\}\},\{3\},\{\{1,2\},3\}\}$

c$.$

$\{\{1,2,3\}\}$

d$.$

$\{3\}$