The correct option is (a) THE
The slogan reads: THE only weapon to fight injustice is education. "THE" is used here as a definite article to indicate a unique, specific weapon.
"THE" is correct because the sentence refers to education as the one and only specific weapon — indicating uniqueness, which requires the definite article "the." Options "a/an" imply one of many, and "this" requires a demonstrative context not present here.