Solution

The series shows a transformation of the inner shape within the square frame.

Pattern Analysis:

The pattern is based on which sides of the outer square the inner shape touches.

• Figure 1 (Hourglass): Touches the top and bottom sides.
  
• Figure 2 (Diamond): Expands to touch all four sides.
  
• Figure 3 (Bow-tie): Contracts to touch only the left and right sides.
  

Conclusion:

The sequence of touching is: Top/Bottom → All Four Sides → Left/Right. The next logical step in this progression is a shape that touches no sides, representing a contraction to the center. The only option that fits this is the circle in figure (a).