For the words you provided:

3. The word that has a different stressed syllable is **B. guitar**. The stress in "guitar" is on the second syllable (gui-TAR), while "listen" (LIST-en), "reggae" (REG-gae), and "concert" (CON-cert) have the stress on the first syllable.

4. The word that has a different stressed syllable is **A. classical**. The stress pattern for the other words is different: "character" (CHAR-ac-ter), "popular" (POP-u-lar), and "conclusion" (con-CLU-sion) typically have stress on the first or second syllable, while "classical" places the stress on the first syllable (CLAS-si-cal).