Cond

(define (leap-year? year)
  (cond
    [(zero? (modulo year 400)) #t]
    [(zero? (modulo year 100)) #f]
    [(zero? (modulo year 4)) #t]
    [else #f]))

The cond form sequentially evaluates a series of test expressions until one produces a non-#f value. Then the associated body for the test expression is evaluated, producing a value. At this point, test expression evaluation inside the cond form ends and the value is returned. If no test expressions are matched, void is produced so often the last expression is #t or else which will always be satisfied, allowing a default value to be set.

The cond form can be visualized as a nested if form.

(define (leap-year? year)
  (if (zero? (modulo year 400))
      #t
      (if (zero? (modulo year 100))
          #f
          (if (zero? (modulo year 4))
              #t
              #f))))

To run though how this works, year is set to 2024, a leap year.

(zero? (modulo year 400)) ; produces #f so ignored
(zero? (modulo year 100)) ; produces #f so ignored
(zero? (modulo year 4)) ; produces #t

The third test expression produces the first non-#f value so its body gets evaluated. That body contains #t which is then the returned value from the cond. That value is then returned by the function.