Which one is most common among terrestrial animals external or internal?

Internal fertilization is the most common mode of reproduction among terrestrial animals.

Here's why:

* Protection from the environment: Internal fertilization protects the eggs and sperm from drying out and other environmental hazards.

* Increased chance of fertilization: Internal fertilization ensures a higher chance of the sperm reaching the egg, increasing the probability of successful reproduction.

* Parental care: Internal fertilization often allows for greater parental care, as the developing offspring are sheltered within the mother's body.

While external fertilization is common in aquatic environments, it is less common on land due to the challenges listed above.