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 โ ababba/ababaa/ababbaย (no particular pattern followed here)
b, a, b โ ababbb/ababaa/ababbbย (no particular pattern followed here)
b, b, b โ ababbb/ababba/ababbbย (no particular pattern followed here)
But,
a, b, a โ ababba/ababba/ababba
Hence, "a, b, a" is the correct answer.