Reproduction

Step 1: Define external and internal fertilization.
- External fertilization: The fusion of male and female gametes (sperm and egg) occurs outside the body of the female.
This typically happens in an aquatic environment where gametes are released into the water.
- Internal fertilization: The fusion of gametes occurs inside the body of the female.

Step 2: Analyze the fertilization type in the given organisms.
Option (1) Rat: A mammal.
Mammals exhibit internal fertilization.
Option (2) Lizard: A reptile.
Most reptiles exhibit internal fertilization.
Option (3) Frog: An amphibian.
Most amphibians, including frogs, exhibit external fertilization.
The female frog lays eggs in water, and the male frog releases sperm over them to fertilize the eggs externally.
Option (4) Cockroach: An insect (arthropod).
Insects typically exhibit internal fertilization.
The male deposits sperm into the female's reproductive tract.

Step 3: Identify the organism with external fertilization.
Among the given options, the frog is the organism that exhibits external fertilization.
This matches option (3).