Answer:
Line E.
Explanation:
The  given program is as follows:
public class Fishing { 
    byte b1 = 4; int i1 = 123456; long L1 = (long) i1;   //Line A 
    short s2 = (short) i1;     //Line B 
    byte b2 = (byte) i1;     //Line C 
    int i2 = (int)123.456;    //Line D 
    byte b3 = b1 + 7;     //Line E 
    }
In the above code Line E will not compile and give following compilation error:
error: incompatible types: possible lossy conversion from int to byte
This error is coming because in Java b1 + 7 will be interpreted as int variable expression. Therefore in order to make code of Line E work the expression should be type casted as byte.  
The correct code will be as follows:
public class Fishing { 
    byte b1 = 4; int i1 = 123456; long L1 = (long) i1;   //Line A 
    short s2 = (short) i1;     //Line B 
    byte b2 = (byte) i1;     //Line C 
    int i2 = (int)123.456;    //Line D 
    byte b3 = (byte)(b1 + 7);     //Line E 
    }