Step 1: Recall the spirit of Total Quality Management.
TQM was developed as a shift away from top down, product driven thinking towards an organisation wide culture where quality is judged by whoever actually uses the product or service.
Step 2: Test each option against this spirit.
A product centred system focuses inward on the item being made rather than outward on who receives it, intermittent improvement contradicts the TQM idea of continuous, never ending improvement, and decisions made only by top executives contradicts TQM's emphasis on total employee involvement at every level.
Step 3: See why customer focus fits perfectly.
TQM holds that the customer, not the producer, ultimately defines what counts as quality, so every process and improvement effort is meant to be judged by whether it satisfies customer needs and expectations.
Step 4: Conclude.
So among the given options, the true TQM principle is customer focus.
\[ \boxed{\text{Customer-focus}} \]