Clinical studies are an essential part of medical research, as they provide crucial information about the safety and effectiveness of treatments. In the given sentence, we are discussing the research on child disease and the status of clinical studies related to it. Let's analyze the options one by one to determine which one is the most appropriate to fill in the blanks.

a) had completed: This option uses the past perfect tense, indicating that the clinical studies were already completed before a certain point in the past. However, the sentence does not provide any specific time frame or point of reference, so we cannot determine if the studies were completed in the past.

b) will be completed: This option uses the future tense, indicating that the clinical studies are yet to be completed. However, the sentence is discussing the current state of the research, so using the future tense would not be appropriate.

c) have completed: This option uses the present perfect tense, indicating that the clinical studies have been completed at some point in the past and have relevance to the present. This option is grammatically correct and conveys the intended meaning in the sentence.

d) had to complete: This option uses the past perfect tense with the modal verb "had to," indicating a necessity or obligation to complete the clinical studies. However, this implies that someone or something required the studies to be completed, which is not mentioned in the sentence.

e) have been completed: This option uses the present perfect passive voice, indicating that the clinical studies have been completed by someone or something. This option correctly conveys the meaning that the studies have been completed without specifying who completed them.

Therefore, the most appropriate option to fill in the blanks is option 'E' - have been completed. It accurately describes the current status of the clinical studies in the child disease research.