Concept:The underlined letter in "laboratory" is the consonant 'b', which is pronounced as the voiced sound /b/.
Explanation:The word "laboratory" contains the letter 'b' with a clear /b/ sound.
Now examine the same letter in each option.
In "thumb", the 'b' is silent, so it does not match.
In "subtle", the 'b' is silent, so it does not match.
In "debtor", the 'b' is silent, so it does not match.
In "dabble", the letter 'b' is pronounced as /b/, which is the same consonant sound as the 'b' in "laboratory".
Answer:D. dabble