How is the excerpt an example of dramatic irony? capulet does not know that his true son-in-law is romeo. capulet expresses his grief by personifying death. capulet is insulting paris who would have been his son-in-law. capulet does not know that juliet is actually alive.

The question given above is incomplete, below is the excerpt attached to the question. 
After Juliet's body is found, Lord Capulet says these lines from act IV of Shakespeare's Romeo and Juliet:
CAPULET: Death is my son-in-law, Death is my heir; My daughter he hath wedded: I will die, And leave him all; life, living, all is Death's. 

ANSWER
The correct option is this: CAPULET DOES NOT KNOW THAT JULIET IS ACTUALLY ALIVE.
A dramatic irony is said to occur, when the audience in a play is aware of a vital fact, which the performing character[s] is not aware of. 
In the excerpt given above, Juliet has drank a sleeping portion in order to avoid getting married to Count Paris, the father, who is not aware of this fact thought that his daughter has actually died. This excerpt is an example of dramatic iron because the audience know that Juliet is not dead, but  Juliet's father is not aware of this fact.