According to triangle law of forces, resultant of two forces must be greater than or equal to the third force.
In option (a),
\(F_1(3 \mathrm{~N})+F_3(1 \mathrm{~N}) < F_2(5 \mathrm{~N})\)
Hence, these sets of concurrent forces may not in equilibrium.
In option (b),
\(F_1(3 \mathrm{~N})+F_2(5 \mathrm{~N}) < F_3(9 \mathrm{~N})\)
Hence, these sets of concurrent forces also may not in equilibrium.
In option (c),
\(F_1(3 \mathrm{~N})+F_2(5 \mathrm{~N}) > F_3(6 \mathrm{~N})\)
Hence, these sets of concurrent forces may be in equilibrium.
In option (d),
\(F_1(3 \mathrm{~N})+F_2(5 \mathrm{~N}) < F_3(15 \mathrm{~N})\)
Hence, these sets of concurrent forces may not in equilibrium.