Concept:Compare the vowel sound of the underlined letters in the target word with the vowel sounds in the options.Explanation:The target word is litmus, pronounced /ˈlɪtməs/.The underlined letter u is pronounced as the unstressed schwa sound /ə/.Option A, papa, is pronounced /pəˈpɑː/.The first a in papa also has the schwa sound /ə/.The other options have different vowel sounds: don has /ɒ/, rust has /ʌ/, and bash has /æ/.Answer:A. papa