Option 1 -> "defer" means to postpone or yield to someone's opinion, which doesn't fit the context of understanding someone's feelings from their tone.
Option 2 -> "refer" means to mention or direct attention to something, which is not appropriate for deducing someone's emotional state.
Option 3 -> "manifest" means to display or show clearly, which would require the subject to actively demonstrate something, not to understand it.
Option 4 -> "infer" means to deduce or conclude from evidence and reasoning, which perfectly fits the context of understanding someone's feelings from indirect cues like tone.
Hence, Option 4: infer -> The speaker is deducing the group leader's unhappiness from the evidence of his tone, which is exactly what "infer" means - to conclude something from available information or clues -> correct