Contextual filter based on base level of taxonomy
If your content type contains a reference field to a taxonomy term with depth, you can use a view block with a normal filter, no contextual filter needed.

Choose 'add filter' &gt; 'Has taxonomy terms (with depth)'
Next screen, select the taxonomy vocabulary (I don't know what it's name is in your case, it should be the vocabulary in which 'option c' is a term) and
set 'Depth' to 1, this will also select the first level of children
Next screen, select the parent term for c1, c2 and c3 (in your case
'option c')

The block will now show all content where the reference field has been used to select option C or it's children.
I used this technique in this site: www.zorgsaamwonen.nl which uses 3 main themes with a lot of child themes.