Masteries are 21 into utility and 9 into defense. The reason for taking 21 points into utility is that it offers supports a lot of different advantages in the game. Cooldown reduction, increase gold, additional start gold and much more. Nine points into defense because it helps you survive the poke and fight during the laning phase, but will also help you later in the game as you won't be able to buy much items (as supports don't farm).
Increased health per level will help you survive the laning phase, but also give you some additional health for late game.
Most of the damage from enemies in bot lane comes from physical damage which armor reduces. Good for laning phase.
Some of the carries in bot lane has some kind of magic damage with their support. Having magic resist help you survive.
30 more health from level one is a huge boost on your survivability. Its a free health quint and will help you survive the laning phase.
Reduced cooldown on Flash will help you in the game overall as you'll have it up more often to play defensive and offensive.
Nami has high mana cost therefore increased mana regen is a good stat on her as you won't run out of mana with this mastery if you don't spam too much.
Reduced cooldown on both flash and exhaust? Sign me in! More offensive and defensive plays.
Reduced cooldown on items help you as you'll be having a lot of items to support your team therefore having them up faster is better.
You're a support and you don't get any income other than gold regen. This mastery will grant you a lot of gold over the duration of a game.
A free health and mana potion in one item to start with helps you as you'll have more money for other important items such as wards. You'll also have more sustain during the laning phase.
Increased start gold allows you to buy additional items which is good as you won't be getting a lot of gold during the game.
A free ward? I'm in! You'll save gold because you won't have to buy as many wards.
Oh I love this mastery. Gold everytime you attack an enemy. Sit in the brush in bot lane and poke with auto attack once in a while to gain gold helps a lot and this mastery will get you rich in no time.
Cooldown reduction to all abilties means more support to your carry as you'll have your abilties up faster to defend him and help him offensive.
Increased movement speed helps you escape and chase enemies. This also helps so you don't need boots as much as you would without.
If you choose to take heal as your summoner spell would I recommend you to change the mastery in defense called resistance
to the mastery called Summoner's Resolve
as it increases the amount of health your heal ability is healing.