
Answer:
750 offspring
Explanation:
The correct answer would be 750 offspring.
The D allele has been labeled with radioactive substance.
If you cross Dd and Dd:
     Dd   x   Dd
   DD  Dd  Dd  dd
3/4 of the resulting offspring would have, at least, the D allele.
Hence, out of 1000 offspring,
   3/4 x 1000 = 750
750 would possess the radioactive label.