Options form with FieldPluginBase?
$this-&gt;options and #options of the select element are two different things.

See the separator field of NumericField::buildOptionsForm:

$form['separator'] = [
  '#type' =&gt; 'select',
  '#title' =&gt; $this-&gt;t('Thousands marker'),
  '#options' =&gt; [
    '' =&gt; $this-&gt;t('- None -'),
    ',' =&gt; $this-&gt;t('Comma'),
    ' ' =&gt; $this-&gt;t('Space'),
    '.' =&gt; $this-&gt;t('Decimal'),
    '\'' =&gt; $this-&gt;t('Apostrophe'),
  ],
  '#default_value' =&gt; $this-&gt;options['separator'],
  '#description' =&gt; $this-&gt;t('What single character to use as the thousands separator.'),
  '#size' =&gt; 2,
];