The correct answer is C. Sevelar
Step-by-step explanation:
In grammar, an adjective refers to a word that describes, qualifies or modifies a noun, for example, in the sentence "The house is blue and big" the words "blue" and "big" are the adjectives as they describe the noun "house". Additionally, an indefinite adjective is a type of adjective that is not specific but mainly general, for example, the words any, not one, each, several, etc. This means, in the case of the sentence presented the indefinite adjective is "several" because this describes "polar explorations" but in an indefinite way as the exact number of explorations is not given.