#### Answer

a. Given any positive real number $r$, the reciprocal of $r$ is positive.
b. For any real number $r$, if $r$ is positive, then the reciprocal of $r$ is positive.
c. If a real number $r$ is positive, then the reciprocal of $r$ is positive.

#### Work Step by Step

Recall that any reference to $r$ in the first half of the sentence must be balanced by a reference to $r$ in the second half. Thus, the following sentence is NOT in proper form: "If a real number $r$ is positive, then the reciprocal of that number is positive."