The correct option is
B frugal
Option B: 'Lavish' means 'rich, elaborate and luxurious'. 'Frugal', on the other hand, means 'simple, plain and costing little'.
As you can see, 'frugal' means the opposite of 'lavish'.
Hence option B is correct.
Option A: 'Expensive' means 'costing a lot of money'.
Option C: 'Wasteful' means 'wasting resources carelessly and without purpose'.
Option D: 'Big' is 'large in size or extent'.
These words do not mean the opposite of the italicized word 'lavish'.
Hence options A, C and D are wrong.