Here in this example question, it says which of the following statements are consistent with Dalton's atomic theory as it was originally stated. So here we're following just Dalton's first five original postulates. We're not taking a look at the modern atomic theory which chemists today use through the use of further experimentations and instrumentation.
So here for one, nitrogen and phosphorus atoms have the same mass. Remember, Dalton wasn't aware of isotopes. He wasn't aware that different elements could have the same mass number and therefore atomic mass, so this would not be consistent when his original posture was. Next, all lead atoms are identical. This is almost word for word one of its postulates. He said that identical elements or identical atoms should possess the same mass, size and properties, so this would be consistent.
Barium and chlorine atoms combine in a one to two ratio to form barium chloride, so Baron chloride is BaCl2. There's one barium to two chlorines. This is consistent with Dalton's original postulate, where atoms combine in simple whole number ratios. Then finally uranium atoms undergo alpha decay to become thorium atoms. This is really talking about radioactive processes which do occur. But again, in Dalton's time he didn't know that this could happen, so this does not go with one of his five original postulates.
So out of the choices, only option two and three could be consistent with Dalton's original atomic theory. So that would give us option C as the correct answer.