Step 1: Find the sentence that introduces the main subject.
Q opens with "Coconut is the major crop cultivated by small and marginal", introducing coconut as the topic and starting a description of the farmers who grow it. This works as the natural opening line.
Step 2: Complete the description of the farmers.
Q ends at "small and marginal", which needs a noun for "farmers", and P supplies exactly that: "Farmers of South India, particularly, Kerala". Together Q and P form one complete sentence about who grows coconut and where.
Step 3: Introduce the difficulty being discussed.
After naming the farmers and their crop, S shifts to a new sentence about a specific challenge: "For every farmer, the practical difficulty in carrying out plant protection in". This logically follows once the farmers and crop are established.
Step 4: State what that difficulty involves.
S ends at "plant protection in", needing an object, and R gives it: "Coconut is the cost and unavailability of laborers", naming the crop again and stating the actual cause of the difficulty.
Final Answer:
The coherent order is Q, P, S, R, matching option (e), folded here as option 4. \[ \boxed{QPSR} \]