Heals and shields are 5% stronger and increased by an additional 10% on targets below 40% health.
Offense
+5 AD or 9 AP
Flex
+5 AD or 9 AP
Defense
+6 Armor
WhySummon Aery
Summon Aery is a great option for Sona both offensively (with autos and [Q]) and defensively (with [W]).
WhyManaflow Band
Manaflow Band is a great option here for Sona because you're going to want to play aggressively in lane early spamming [Q].
WhyCelerity
Celerity's bonus Movement Speed is great to for helping you to position better in teamfights so that you aren't quickly dying from your low health pool.
WhyScorch
Scorch allows you to dominate the lane phase if you're able to consistently poke with your [Q].
WhyBone Plating
Bone Plating on Sona provides fantastic early-game mitigation so that you're able to take good trades with [Q] poke and an empowered Auto Attack.
WhyRevitalize
Revitalize synergizes extremely well with Sona's [W] especially heading into the mid to late game when you've built a few extra heal/shield items.