Boundaries of solids are:
(A) surfaces
(B) curves
(C) lines
(D) points
(A) surfaces
Solution:
Solid: A three-dimensional object having a definite shape, size and volume is called a solid.
Surface: A surface is the outermost layer of a physical object (solid) or we can say that surface is a kind of a plane.
Hence, the surfaces are the boundaries of solids.
Hence option (A) is correct.