Answer :- Estrogen alone.
Hysterectomy:-A hysterectomy refers to the surgical removal of a woman’s uterus. It may be performed for a number of reasons, both benign (for example, uterine fibroids) and cancerous (such as uterine cancer).
Depending on the reason behind why a hysterectomy is being done, a doctor may also remove the ovaries and fallopian tubes (the tubes that connect a woman's ovaries to her uterus).
With the removal of the ovaries, a premenopausal woman will immediately go into menopause, called surgical or induced menopause. Since she no longer has ovaries to produce estrogen, she may experience classic symptoms of estrogen depletion like hot flashes and/or vaginal dryness.
Estrogen plays a key role throughout the body. It affects the brain, the bones, the skin, the heart, the blood vessels, and more. While estrogen levels lower gradually during natural menopause, they plummet with surgical menopause.That sudden drop in estrogen can lead to menopausal symptoms that can be quite severe.
Hormone therapy after surgery -- with estrogen alone -- is a way to counteract the supply of estrogen you've lost.
Women who have both the uterus and ovaries removed usually just get estrogen replacement therapy (ERT) alone. But women who have only the ovaries removed need both estrogen and progestin.
That's because estrogen alone can increase the risk of cancer in the uterus. Adding progestin removes this risk.