"Replace we’re with were" is the correct answer. This is correct because we’re is a contraction of we and are, while were is a verb.
"Replace relieved with releived." This is incorrect because of the I before E rule.
"Replace exhausted with exausted." This is incorrect because the h in exhausted is silent.
"Replace firefighters with firefighter’s." This is incorrect because firefighters is a plural noun, whereas firefighter’s is a possessive noun.