Pulmonary vascular resistance is reduced by:

Acutely increasing pulmonary venous pressure will raise capillary pressure and result in recruitment and distension of the capillaries. The other choices are incorrect because removing one lung greatly reduces the vascular bed, 10% oxygen breathing results in hypoxic pulmonary vasoconstriction, reducing lung volume to residual volume increases the resistance of the extra-alveolar vessels, and mechanically ventilating the lung with positive pressure increases the alveolar pressure and therefore tends to compress the capillaries.