A variable is live at a particular point in the program if its value at that point will be used in the future (dead, otherwise).
Apply this definition,
In block 1, variables a,c,d are present, here the value which each of the variables among a,c,d will have, if that same value is used in further operation then those variables are live. Note that the same value which is initialized in the present block must be used in further blocks. If you notice this then rest is easy. So, option D is the correct answer.
Reference: https://www.cs.columbia.edu/~suman/secure_sw_devel/Basic_Program_Analysis_DF.pdf
