Following spinal subarachnoid block a patient develops hypotension. This can be managed by the following means except –
This is the controversial one. Most of the guides have given option 'a' i.e. lowering the head end as the answer. Their logic is that, if the head end is lowered, the anesthetic drug will move up and will produce severe hypotension and bradycardia.
However, all texts books have mentioned that in the management of hypotension after spinal anesthesia, head low position (Trendelenburg position) should be given to increase the venous return.

"Head low position (Trendelenburg position) to increase the venous return. Although the concern of trendelenburg position increases the chances of high spinal anesthesia, but number of studies have proven that the beneficial effects over weigh these problems and secondly head low up to 15° does not significantly increase the level of block. So, trendelenburg up to 15° does not significantly increase the level of block. So, trendelenburg up to 15° is still the very vital part of management for the treatment of spinal hypotension". 
— Fundamentals of anesthesia
"Autotransfusion may be accomplished by placing the patient in head down position".                                             
— Morgan 4th/e 297
"Positioning the operating room table in head down position following administration of spinal anesthesia does not significantly affect the hypotension incidence".
— Evidence based obstetric anesthesia 1st/e 100
About option b

Preloading the patients with colloids prevent hypotension. Preloading with crystalloid does not prevent hypotension because large volumes of crystalloids quickly redistribute from intravascular to extravascular space.

"Studies of large volumes (12-30 ml/kg) of crystalloid infusion before spinal anesthesia have shown no difference in the incidence of hypotension probably because large volumes of crystalloid quickly redistribute from the intravascular to the extravascular compartment".
—Lee 13th/e 509
"In contrast to crystalloid solutions, prophylactic volume loading with colloid solutions (500-100 ml) prior to spinal injection has consistently maintained blood pressure and volume expansion in both surgical and obstretic patients owing to a sustained increase in intravascular volume, maternal cardiac output and uteroplacental blood flow".
— Lee 13th/e 510
"Combining a low-dose spinal anesthetic technique with colloid preloading minimizes the hemodynamic consequences and vasopressor requirements of neuraxial anesthesia of cesarian section". 
—Lee 13th/e 510
"That the decrease in arterial blood pressure after neuraxial block can be minimized by the administration of crystalloid intravenously is probably is not a valid concept".
—Miller 7th/e 1617
"Although crystalloid preload is commonly administered before spinal anesthesia for cesarian section, the effect on the incidence of hypotension is not considerable. The use of colloid solutions is more efficacious".
— Evidence based obstetric anesthesia 1st/e 100
"Colloids are better than crystalloids for preloading to prevent hypotension".
— Perioperative fluid therapy 1"/e 346
Note :-

Despite of above facts, crystalloids are commonly used.
Lowering the head end
Administration of 1000 ml of Ringers lacate before the block
Vasopressor drug like methoxamine
Use of ionotrope like dopamine