Which word is formed by combining 'उत् + हार'?

According to the rules of Consonant Sandhi (व्यंजन संधि), when 'त्' or 'द्' is followed by 'ह', 'त्' changes to 'द्' and 'ह' changes to 'ध्'.
Therefore, $\text{उत्}+\text{हार}=\text{उद्धार}$.