Option 1 states that a true fruit is formed when only the thalamus of the flower grows and develops into a fruit. However, this is incorrect. The thalamus of the flower, also known as the floral axis or the receptacle, can contribute to the structure or shape of the fruit, but it alone cannot be considered a true fruit.

Option 2 suggests that a true fruit is formed when only the receptacle of the flower develops into a fruit. This is also incorrect. The receptacle can play a role in fruit development, but it is not sufficient to form a true fruit on its own.

Option 3 states that a true fruit originates only from the calyx of a flower. This is inaccurate as well. The calyx is the outermost whorl of a flower and comprises sepals. While the calyx may protect or enclose the developing fruit, it does not directly contribute to the formation of a true fruit.

Option 4, the correct answer, states that a true fruit is formed when only the ovary of the flower grows into a fruit. This is true. The ovary contains the ovule, which upon fertilization, develops into a seed. The ovary wall then undergoes changes and forms the fruit.