A longitudinal crack in the center of a root pass is likely caused by:

A longitudinal crack in the center of a root pass can indeed be caused by both improper preheating and high restraint pressure of the workpieces.

Improper preheating can lead to thermal stresses that occur during the welding process. When the metal is heated unevenly or not adequately preheated, it can cool too quickly, leading to brittle zones that are prone to cracking. The root pass, being the first weld layer, is particularly susceptible to these conditions, as it is fully exposed to the thermal gradients during the welding process.

High restraint pressure of the workpieces means that during the welding operation, the components are under significant mechanical stress due to their configuration or clamping. This restriction can further exacerbate the likelihood of cracking as the weld metal tries to expand and contract with temperature changes. When a weld is subjected to such high restraint, it may result in tensile stresses that can cause cracks to form along the weld line.

Thus, both the improper preheat and the high restraint are contributing factors, making the correct answer the combination of the two.