Which of the following pairs of receptors are likely to show the greatest structural similarity?

Which of the following pairs of receptors are likely to show the greatest structural similarity?
a) the dopamine receptor subtypes D3 and D5
b) the M2 muscarinic receptor and the β2-adrenergic receptor
c) the H2 histamine receptor and the α1-adrenoceptor
d) the H1 histamine receptor and the β2 adrenoceptor