The correct option is
A baked
The verbs that do not explain the tense of the sentence and do not agree with the subject are called non-finite verbs. Non-finite verbs are usually infinitives, gerunds and participles.
Option A - 'Baking' is the participle form of the verb. If the cookies are being baked, then it is not possible for one to know whether it's delicious. Hence, option A is incorrect.
Option C - 'To bake' can't be used here as the sentence is in the present tense, which means that the cookies are already baked. Hence, option C is incorrect.
Option D - 'Bake' is the original form of the verb. Also, as the sentence is in simple present tense, we can't use 'bake cookies', as that would sound like an order. Hence, option D is incorrect.
Option B - 'Baked' is the past form of the verb, but it doesn't describe the sentence in past tense. It is a non-finite verb here. Hence, option B is the correct answer.
'The freshly baked cookies are delicious.'