apply_filters( 'relevanssi_allow_one_letter_highlights', false )
Return true to allow Relevanssi add one-letter highlights to excerpts.
Parameters
$allow
(boolean) If true, set the minimum word length for excerpt highlights to 1.
More information
By default Relevanssi ignores one-letter search terms and does not add highlights that are just one letter. To enable the one-letter search terms, the relevanssi_block_one_letter_searches needs to return false and then you can have this filter return true to see one-letter highlights.
add_filter( 'relevanssi_allow_one_letter_highlights', '__return_true' );