Answer:
Step-by-step explanation:
"the fate of the diamond, along with several other high profile gems, were unknown" - subject-verb disagreement between "fate" (singular) and "were" (plural)
"the twin eighty-year-old brothers suggests" - subject-verb disagreement between "brothers" (plural) and "suggests" (singular)
"It will be on display for four weeks only." - subject-verb disagreement between "It" (singular) and "will be" (plural)