Forces in equilibrium form a closed vector triangle

Steps:

• Identify forces on frame: weight $Mg$ downward, two equal tensions $T$ along strings upward at symmetric angles to vertical.
• For equilibrium, vector sum is zero: tensions balance weight.
• Construct triangle: place $Mg$ vertical; add two $T$ vectors head-to-tail at angles to oppose it.
• Closed triangle shows resultant zero, with $T$ magnitudes equal and angles matching setup.

Why D is correct:

• D depicts closed triangle with vertical $Mg$ base and two equal $T$ sides at acute angles, satisfying Lami's theorem for concurrent forces in equilibrium.

Why the others are wrong:

• A: Open triangle, forces do not close, violating equilibrium condition.
• B: Unequal tension vectors, inconsistent with symmetric string setup.
• C: Horizontal components unbalanced, tensions not symmetric to vertical weight.

Final answer: D