。
What is your reason for wanting to enroll at this university?
Literal
This-university [target-に] enroll-want reason [topic-は] what [is-です]-[question-か]?
本学 ('this university,' a formal institutional self-reference) is typical of admissions and formal school settings. 入学したい理由 ('the reason (one) wants to enroll') is a short relative clause modifying 理由 — the ~たい-form verb attaches directly to the noun with no の, because verbs modify nouns directly in Japanese. A standard interview or application prompt.