Step 1: Understand linguistic properties of sign languages.
Modern linguistic research establishes that sign languages are full-fledged natural languages with systematic grammatical structure.
Step 2: Evaluate option (A).
Sign languages display duality of patterning, where meaningless units such as handshape, movement, and location combine to form meaningful signs. This makes option (A) correct.
Step 3: Eliminate incorrect options.
(B) While some signs may be iconic, most are arbitrary.
(C) Deaf children can spontaneously develop structured sign systems even without formal instruction.
(D) Sign languages are not pidgins but fully developed languages.
Step 4: Conclusion.
Thus, the correct answer is option (A).