Which statement correctly expresses the triangle law of vector addition?
(a) If two vectors are represented by two sides of a triangle taken in order, the third side taken in the same order represents their resultant.
(b) Two equal vectors placed tail to tail always form a parallelogram whose diagonal is twice their magnitude.
(c) If two vectors are represented by two sides of a triangle taken in order, the third side taken in the opposite order represents their resultant.
(d) The magnitude of the resultant of two vectors is always the sum of their magnitudes, irrespective of the angle between them.