In reference to the ad placed in today's newspaper, several callers ex

The best answer is C.

This question can be answered with two splits, which are based on idiomatic construction.

Split #1: In reference to vs with reference to vs referring to vs when referring to

In reference to is unidiomatic. With reference to, when referring to, and Referring to are correct idiomatic forms. We can eliminate A based on this.

Split #2: interest in vs interest about
Interest about is unidiomatic. To correct idiomatic form is to express interest in. B, D, and E can be eliminated based on this.

C is left as the best out of the lot.

In reference to the ad placed in today's newspaper, several callers expressed interest in renting the currently empty loft.

A. In reference to the ad placed in today's newspaper, several callers expressed interest in
B. Referring to the ad placed in today's newspaper, callers expressed interest about
C. Referring to the ad placed in today's newspaper, callers have expressed interest in
D. With reference to the ad placed in today's newspaper, callers frequently expressed interest about
E. When referring to the ad placed in today's newspaper, callers frequently express interest about

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