It's only because you know The simple past is a verb tense that is used to talk about things that happened or existed before now.

Explanation:
When writing a diary entry to describe something that happened yesterday, the most appropriate tense to use is the past simple tense (option C). Here's an explanation of why the past simple tense is the correct choice:

Past Simple Tense:
The past simple tense is used to describe completed actions or events that happened at a specific time in the past. It is formed by adding '-ed' to regular verbs or using irregular verb forms. In the case of a diary entry, the past simple tense is used to reflect on and recount events that occurred in the past, such as yesterday.

Example:
Let's take an example of a diary entry to illustrate the use of the past simple tense:

Dear Diary,


Yesterday was a busy day for me. I went to school in the morning and attended my classes. After school, I met up with my friends at the park. We played soccer for a while and then enjoyed a picnic. It was a lot of fun! Later in the evening, I helped my mom with dinner and then watched my favorite TV show. I had a great day overall.

In the example above, all the verbs used to describe the events that took place yesterday are in the past simple tense. This helps to convey that the events happened in the past and are now being reflected upon in the diary entry.

Using any other tense, such as the present simple (option A) or present continuous (option B), would create confusion and make the diary entry sound unnatural. The future simple tense (option D) would not be appropriate since we are talking about events that have already taken place.

Therefore, the correct tense to describe something that happened yesterday in a diary entry is the past simple tense (option C).