Step 1: Identify the opening connector.
The sentence begins with “However the major drawback”, which must logically be followed by a phrase explaining what the drawback is.
Part P (“of this process is that”) correctly completes this opening.
Step 2: Identify the core idea.
After stating the drawback, the sentence should explain what the drawback actually is.
Part Q (“there is no legally binding”) introduces the key issue.
Step 3: Complete the meaning.
Part R (“outcome at the end of six years”) logically completes the statement begun in Q.
Step 4: Form the complete sentence.
Arranging the parts as P → Q → R, the complete sentence reads:
\textit{“However the major drawback of this process is that there is no legally binding outcome at the end of six years.”}