Explanation:

The correct preposition to use in this sentence is "in". Let's break down the reasons why "in" is the correct choice.

1. Location:
The word "picnic" implies an outdoor activity, and in this case, it is specified that the picnic will take place "in the park". The preposition "in" is used to indicate that something is located or situated within an enclosed or defined space. In this context, the park is the defined space where the picnic will occur.

2. Enclosed Space:
The preposition "in" is commonly used when referring to locations that are enclosed or surrounded by boundaries. In this case, the park is an enclosed space with definite boundaries, such as a fenced area or designated park grounds. Therefore, "in" is the appropriate preposition to indicate that the picnic will take place within the boundaries of the park.

3. Specificity:
Using the preposition "in" adds a level of specificity to the sentence. It clarifies that the picnic will not just be near or around the park, but rather inside the park itself. This choice of preposition ensures that the reader understands the exact location of the picnic.

4. Common Usage:
The phrase "have a picnic in the park" is a commonly used expression in English. It is the standard way of describing the act of having a picnic within a park setting. By using the preposition "in", the sentence follows the established conventions and idiomatic usage of the English language.

In conclusion, the correct preposition to use in the sentence "We will have a picnic __________ the park" is "in". This choice accurately conveys the location of the picnic within the boundaries of the park and aligns with the standard usage of the English language.