What do the branch lengths of a phylogenetic tree represent under the default display of the

"Viewer" tab of the PhyML Newick tree $($here phylogram$)?$

$($a$)$ no meaning.

$($b$)$ proportional to the distances in the underlying distance matrix $($~genetic change$).$

$($c$)$ proportional to time.

$($d$)$ proportional to bootstrap values