The correct option is A 12[sec x tan x+log(sec x+tan x)]
Let I=∫sec3x dx=∫secx sec2x dx
⇒I=sec x tan x−∫sec x tan2x dx⇒I=sec x tan x−∫sec x(sec2x−1)dx⇒I=sec x tan x−∫sec3x dx+∫sec x dx⇒I=sec x tan x−I+log(sec x+tan x)⇒2I=sec x tan x+log(sec x+tan x)⇒I=12[sec x tanx+log(sec x+tan x)]