Question

Based on this etymological information, what is the most likely meaning of the word endolithic?
monolith: Greek, mono "single, alone" + lith "stone"
endocrine: Greek, endo "inside" + krinein "to separate, distinguish"
extrinsic: Latin, exter "outside" + in (suffix of locality) + secus "beside"
endotoxic: Greek, endo "inside" + Latin toxicum "poison"

a single outer surface made of stone

living inside a stony substance

poisonous from inside

a stone belonging to an ancient period