Which of the following secretes the hormone, relaxin, during the later phase of pregnancy ?

The correct option is Corpus luteum.

Step-by-Step Explanation:

1. Understanding Relaxin: Relaxin is a peptide hormone that plays a crucial role in the female reproductive system and during pregnancy. Its primary function is to relax the ligaments in the pelvis and soften and dilate the cervix to facilitate childbirth.

2. Source of Secretion: During the later phase of pregnancy, relaxin is secreted in significant amounts by the corpus luteum of the ovary. While the placenta also produces relaxin, the corpus luteum remains a major source of this hormone during gestation.

3. Analyzing the Options:
- Graafian follicle: This is the mature ovarian follicle that releases the ovum during ovulation. It primarily secretes estrogen but is not the source of relaxin during pregnancy.
- Corpus luteum: Formed after ovulation from the ruptured Graafian follicle, it secretes progesterone and also secretes relaxin in the later stages of pregnancy.
- Foetus: The developing offspring does not secrete relaxin.
- Uterus: While the uterus is the target organ for many pregnancy hormones, it is not the endocrine gland that secretes relaxin.

Therefore, the corpus luteum is the correct endocrine structure responsible for secreting relaxin during the later phase of pregnancy.