Solution:We put all the option in blank area and check who create particular pattern
a, b, a, b, b, _ b, a, b, _ a, a, b, b, _
a, a, a → ababba/ababaa/ababba (no particular pattern followed here)
b, a, b → ababbb/ababaa/ababbb (no particular pattern followed here)
b, b, b → ababbb/ababba/ababbb (no particular pattern followed here)
But,
a, b, a → ababba/ababba/ababba
Hence, "a, b, a" is the correct answer.