The efficient cause is "the primary source", or that from which the change under consideration proceeds.
ðɪ ɪfˈɪʃənt kˈɔːz ɪz "ðə pɹˈaɪmɚɹi sˈoːɹs", ɔːɹ ðæt fɹʌm wˈɪtʃ ðə tʃˈeɪndʒ ˌʌndɚ kənsˌɪdɚɹˈeɪʃən pɹˈoʊsiːdz.