Which of the following series is in non-increasing order?

A sequence a1, a2, a3, …, an is said to be in non-increasing order if every term is less than or equal to the term immediately before it — that is, a1 ≥ a2 ≥ a3 ≥ … ≥ an. Consecutive terms are allowed to repeat; the sequence must simply never rise.

Checking each series against this rule:

• 1, 3, 4, 6, 8, 9 — every term is strictly greater than the one before it, so this series is increasing, not non-increasing.

• 9, 8, 6, 3, 4, 1 — the terms fall from 9 to 3, but then rise to 4 before falling to 1, so the series is not non-increasing.

• 9, 8, 6, 3, 3, 1 — every term is less than or equal to the one before it throughout, so this series is non-increasing.

• 1, 3, 3, 6, 8, 9 — every term is greater than or equal to the one before it, so the series is non-decreasing, the opposite pattern.

Scanning every consecutive pair confirms only one series never rises anywhere — that is the one satisfying the non-increasing definition.