The correct dative plural of dux is ducibus. As a third declension noun with a genitive of ducis, the proper ending, -ibus is added to the stem, resulting in ducibus.

The correct answer is "puero". It should be rememebered that Mandare in Latin takes the dative case instead of the accusative; therefore, "puero" is the only viable option.

The verb mandare takes the dative instead of the accusative; therefore the only correct answer can be militibus; since every other option is in the wrong case.