Question

In a chloroplast, the stacks of membranous sacs are called

Answer 1

Grana is part of the thykaloid. Each stack of thykaloid (the individual disks) are called grana.Cristae are those twisted, curving-shaped inner membranes in mitochondria where cellular respiration occur.Vacuoles stores food, waste, etc.
Stroma is the fluid-like part in the chloroplast. Thykaloid is contained in the stroma. 

So, are the membranous sacs called thykaloid.