Concept:The consonant sound represented by the underlined letters is the voiceless fricative /f/.
Explanation:We compare the pronunciation of each option to find the one containing /f/.
The word "tough" is pronounced /tʌf/, so it clearly ends with the matching /f/ sound.
The word "approve" ends with the voiced sound /v/, not the voiceless /f/.
Similarly, "believe" ends with /v/, so it does not match.
The word "thorough" is pronounced /ˈθʌrə/ and has no /f/ sound anywhere in it.
Therefore, only "tough" shares the same consonant sound as the underlined letters.
Answer:B. tough