The correct answer is Option (D).
It is a beautiful day for a picnic by the lake in the central park is a complete and correct sentence. It explains when an action takes place that is today which is regarded a “beautiful day” and where the action is to take place “by the lake in Central Park”.
Option A (it is the beautiful day) is not correct because of the word “the” it makes the sentence an unspecified sentence.
Option B is also wrong but confusing. “Let’s go on a picnic” sounds like a complete statement but it’s not definite of where and when this action should take place.
Option C is also incorrect
It is a beautiful day for a picnic by the lake in central park.-a is not correct because beautiful day is unspecific. you could use the in this sentence if you indicated which beautiful day you meant, as in:it is the beautiful day i hoped for yesterday.b is not correct because it does not include the indefinite article a before park.c is incorrect. the a is used correctly before lake but you dont need to use an article before a proper noun such as central park because it is a name of a specific park. you could use the in the sentence if you removed the name of the park, as in:is there a lake in the park?d is correct.