Choose the next diagram in the given figure series:

Let's trace the shift pattern of the elements across the sequence steps:
1. The triangle ($\Delta$) moves in a strictly clockwise direction from corner to corner at each step: bottom-left $\to$ top-left $\to$ top-right $\to$ bottom-right $\to$ bottom-left. Following this cycle, the next position for the triangle in Box 6 must be the top-left corner.
2. The square/rectangle ($\square$) rotates in a complementary rhythmic fashion.
3. Looking at the options, only Option A correctly positions the triangle at the top-left corner.