Concept:A verb form is chosen based on the time clue and structure of the full sentence.
Explanation:The sentence containing the blank is not shown in the question.
Without that sentence, no single option can be chosen as the best answer.
Each option is grammatically correct in a different context.
Option A, slide, is the base form used in the present tense.
Option B, slid, is the simple past form.
Option C, had slid, is the past perfect form.
Option D, was sliding, is the past continuous form.
The words around the blank would normally indicate which tense is required.
Since those words are missing, more than one answer is possible.
Answer:Cannot be determined because the sentence to be completed is missing.