What did Justinian I fail to regain all of the former provinces of the old Roman Empire

The correct answer is C. He ran out of money.

Justinian I failed to regain all of the former provinces of old Roman Empire because he ran out of money. His once powerful army was weakened by the plague and the finances were down. He ran out of options.

Justinian I or Justinian the Great was the Eastern Roman Emperor from 527 to 565.