Solutions
The meanings of the given idioms are as follows:
A. Couch potato- a lazy person who spends a lot of time sitting down or lying down.
B. Down- to- Earth- a practical, sensible, and realistic person.
C. Chip off the old block- someone who is very similar in character to their father or mother.
D. Woman of her word- a person you can trust because they tell the truth and keep promises.
E. Wet blanket- a person who discourages fun or enjoyment for other people.
The sentence implies that the father and son were of opposite nature even when the son considered otherwise.
The other idioms are less likely to fit as they would not represent the relationship between the first and the second sentence. "Down-to-Earth" must be used as an adjective (that is, it must be followed by a noun like "person") and not as a noun.
So, the correct answer is option C.