The prefix “ob-“ means

a: with
b: against
c: away
d: not

a with

Explanation:ob- WORD ORIGIN. a prefix meaning “toward,” “to,” “on,” “over,” “against,” originally occurring in loanwords from Latin, but now used also, with the sense of “reversely,” “inversely,” to form Neo-Latin and English scientific terms: object; obligate; oblanceolate.

B. Against

The English prefix ob- usually means “against, in the way” or “towards.” An example of ob- when it means “towards” is the word obligation, or a tying of some duty “towards” you; in the case of ob- meaning “against” or “in the way,” the word obstacle" refers to that which stands “against” you, “in the way” of proceeding from point A to point B.