The correct option is
C existent
In this sentence we would be choosing a word alternate to the given word, "Extant"
Extant means still in existence; surviving.
Existence means the fact or state of living or having objective reality.
The above two words are synonymous.
If we look into the other options,
Hidden means kept out of sight; concealed
Scattered means occurring or found at intervals or various locations rather than all together.
Spread means open out (something) so as to extend its surface area, width, or length.
Option C is correct