Senna Runes explained
There's one set of runes for Senna on patch 14.23 that is the clear favorite for LoL players and pros alike. The best Senna rune paths on the position 
Support are 
Resolve as the primary path and Precision as the secondary path with a pick rate of 37%. Besides picking runes on Senna you should learn how to play Senna and if you are interested in pro matches take a look on how a pro builds Senna. If you want to discuss 
Senna then you should visit the champion subreddit.
Senna runes from primary path
Resolve empowers the survivability of Senna and increases the tankiness. Most of high elo players – and our LoL pros in the the major regions – have agreed that Grasp of the Undying is the best keystone rune to run on Senna. This keystone rune gives Senna alot of sustain and helps to force your opposing champion out of the lane. Other primary runes on Senna are Font of Life, Bone Plating and Revitalize.
Font of Life provides your teammates with more sustain by healing them whenever they attack a champion whose movement you've recently impaired. Bone Plating make Senna overall tankier, by decreasing the amount of poke damage in the early game. Revitalize inceases heals and shields you distribute or receive.
Senna runes from secondary path
When it comes to your secondary path, most Senna players prefer to go with Precision. Precision improves attack and sustained damage on Senna troughout the match. On the Precision path, the best runes for Senna are Presence of Mind and Legend: Alacrity.
Presence of Mind helps Senna with the mana problems by restoring missing mana, if a champion is killed. Legend: Alacrity grants additional attack speed to Senna.
FAQ for Senna Runes
Which is the best keystone rune on Senna?
The best keystone rune on Senna is Grasp of the Undying. This Senna runes setup is then referred as Senna Grasp of the Undying.
What runes does Senna use?
Runes from the paths Resolve and Precision are the most used runes on Senna. For the exact pick rates of Senna runes used, you have to study the rune distribution presented.