Step 1: Understand informal organisation:
An informal organisation is a network of social relationships that grows on its own among employees. It comes from friendship, common interests and shared habits. It is not created by the management.
Step 2: Check Assertion (A):
The assertion says informal organisation has a definite structure. This is wrong.
It has no fixed form or written rules. No chart shows who stands where. The groups can form and break at any time.
A definite structure belongs to the formal organisation. So (A) is FALSE.
Step 3: Check Reason (R):
The reason says informal organisation is a complex network of social relationships among members. This is correct.
People talk, share lunch and help each other, and this builds a web of relationships. So (R) is TRUE.
Step 4: Check the options:
(A) Both true, with R explaining A: wrong, because A is false.
(B) Both true, but R does not explain A: wrong, because A is false.
(C) A true and R false: wrong, because A is false and R is true.
(D) A false and R true: matches our findings.
Final Answer:
Assertion (A) is false and Reason (R) is true, so the correct option is (D).