Answer
$1L$ of $0.20M$ $NaOH$ and 0.10 mol $NaCH_3COO$ wouldn't form a buffer, because there is no acid to neutralize added bases.
Work Step by Step
$- NaOH:$ Strong base.
$- NaCH_3COO$: Weak salt base.
A buffer needs:
1. An acid and a base:
Base: $NaOH$ and $NaCH_3COO$
Acid: Missing.
Therefore, this mixture can't act as a buffer, because it doesn't have an acid, so it can't neutralize added bases.
