FAG, GAF, HAI, IAH, _____
$\textrm{Option A.}$
$\textrm{First term like F,G,H,I,J...}$
$\textrm{Second term remain unchanged.}$
$\textrm{Third term follows like -1,+3,-1,+3}$
Ref: https://www.indiabix.com/logical-reasoning/letter-and-symbol-series/discussion-85