The correct answer is choice A. Retained placental fragments are the most common cause of subinvolution. Subinvolution is a condition where the uterus does not return to its normal size after childbirth. Retained placental fragments prevent the uterus from contracting properly and cause prolonged bleeding and infection.

Choice B is wrong because infection is not the most common cause of subinvolution, but it can be an aggravating factor. Infection can cause inflammation and interfere with the healing of the uterine lining.

Choice C is wrong because uterine fibroids are not the most common cause of subinvolution, but they can be a predisposing factor. Uterine fibroids are benign tumors that can distort the shape of the uterus and impair its contraction.

Choice D is wrong because multiparity is not the most common cause of subinvolution, but it can be a predisposing factor. Multiparity means having given birth more than once, which can weaken the uterine muscles and reduce their ability to contract.

Normal ranges for uterine involution are as follows:

• Uterus weight: decreases from about 1000 g at delivery to about 60 g at six weeks postpartum.

• Uterus height: decreases from about 20 cm above the pubic bone at delivery to about 12 cm at one week postpartum, and then descends into the pelvis by six weeks postpartum.

• Uterus size: decreases from about 20 times its normal size at delivery to about its normal size at six weeks postpartum.