In what way were the firstborn son of a knight and the firstborn son of a serf alike? Both children would grow up to have the same roles as their fathers. Both children had to prove they were worthy of their position in society. Both children took a vow of loyalty to their lords, just as their fathers did. Both children could choose their own occupation independent of their parents.
"Both children would grow up to have the same roles as their fathers" is the best option from the list, since there was practically no upward social mobility for serfs during this time.