Explanation:

- Option 1: cost

- This is the correct choice. The word "cost" is used in the simple present tense, which is suitable for stating the price of something in general terms.

- Correct: These apples cost Rs.100 per kg.

- Option 2: is costing

- This form, present continuous, is generally used for ongoing actions. However, it is not suitable for stating prices, which are usually expressed in simple present tense.

- Option 3: has cost

- This is the present perfect tense. It is used for actions that occurred at an unspecified time, not for stating current prices.

- Option 4: costs

- This could be acceptable if "apples" were singular, like "apple costs," but since we are talking about multiple apples, this doesn’t fit.