C. Pre-eclampsia: This condition is characterized by the onset of hypertension and often proteinuria after 20 weeks of gestation in a previously normotensive woman. However, it can occur earlier, especially in women with a history of pre-eclampsia or those carrying multiple fetuses. The patient's symptoms of headaches and blurred vision, combined with significantly elevated blood pressure (180/110 mmHg) at 15 weeks gestation, strongly suggest pre-eclampsia, especially considering the presence of symptoms that could indicate severe features.

B. Pregnancy-induced hypertension: This is a broader category that includes pre-eclampsia but can also refer to gestational hypertension without proteinuria or other signs of pre-eclampsia. The patient's high blood pressure and gestational age could fit this diagnosis, but the presence of symptoms like headaches and blurred vision leans more towards pre-eclampsia.
A. Essential hypertension: Although less likely given the acute onset and severity of symptoms in the context of pregnancy, essential hypertension could be considered if the patient had a history of hypertension before pregnancy. However, the significant elevation in blood pressure and the presence of symptoms such as headaches and blurred vision at this stage of pregnancy make this less likely.

D. Pre-eclampsia with severe features: This is a critical diagnosis to consider given the patient's blood pressure is significantly elevated (180/110 mmHg), and she is experiencing symptoms such as headaches and blurred vision, which can indicate severe pre-eclampsia. Missing this diagnosis could lead to severe maternal and fetal complications, including eclampsia, stroke, and placental abruption.
Chronic hypertension with superimposed pre-eclampsia: Although not listed among the options, it's crucial to consider this condition, especially if the patient has a history of hypertension. The superimposition of pre-eclampsia on chronic hypertension can lead to more severe outcomes.

Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodic or sustained hypertension. Although rare, it's a diagnosis that should be considered in cases of severe or resistant hypertension, especially if accompanied by other symptoms like palpitations, sweating, or headache.
Secondary hypertension due to other causes: Other rare causes of secondary hypertension, such as renal disease, primary aldosteronism, or coarctation of the aorta, could potentially present with severe hypertension during pregnancy. However, these would be less common and might require additional symptoms or findings to suspect.