This word means the same as the given italicized word. Hence option C is correct.
Option A: 'Strict' means 'demanding total obedience or observance, rigidly enforced'.
Option B: 'Economic' means 'making a profit, or likely to make a profit'.
Option D: 'Miserly' means 'pitiably small or inadequate'.
These words do not mean the same as the given word 'frugal'.
So, options A, B and D are wrong.