Answer:
A. three planes intersecting at a point
Explanation:
Whenever three planes intersect at a point, then we have an ordered triplet (x,y,z) that is a solution to the associated system of equation.
This is a unique solution and it is finite.
However three planes intersecting at a line gives infinitely many solutions.
Three or two parallel planes may have no solution or or infinitely many solutions when they coincide.