Question 18, Bio/Biochem, Practice Test 3 (2018)

Please help me understand why two isoforms of a protein are produced by a single mRNA transcribed from a single promoter sequence upstream of the operon. I'm looking this up in wikipedia and I can't understand why it's not 2 different mRNAs.
Source: Student Doctor Network - Category: Universities & Medical Training Authors: Tags: MCAT: Medical College Admissions Test Source Type: forums