Step 1: Cod fish
Cod fish, like many cold-water fish, is indeed a rich source of $\omega$-3 fatty acids (EPA and DHA). So (A) is correct.
Step 2: Beetroot
Beetroot is rich in betalain pigments (betacyanins and betaxanthins), not in $\beta$-carotene. Therefore, the statement that beetroot is a good source of $\beta$-carotene is incorrect.
Step 3: Carrots
Carrots are famously rich in $\beta$-carotene, the precursor of vitamin A. So (C) is correct.
Step 4: Spinach
Spinach contains both iron and folate, making (D) correct.
Hence, the NOT correct statement is (B) Beetroot is a good source of $\beta$-carotene.