When both jaws of vernier callipers touch each other, zero mark of...

Given Least Count ($LC$) = $0.1$ mm.
When jaws touch, the zero of the vernier scale is to the right of the main scale zero, indicating a positive zero error.
Zero error ($e$) = $+ (n \times LC)$, where $n$ is the coinciding vernier division.
$e = +(4 \times 0.1) = +0.4$ mm.
While measuring the cylinder:
Main Scale Reading ($MSR$) = $15$ divisions.
Assuming $1$ MSD = $1$ mm (standard for $LC = 0.1$ mm), $MSR = 15$ mm.
Vernier Scale Reading ($VSR$) = $5 \times LC = 5 \times 0.1 = 0.5$ mm.
Observed Reading = $MSR + VSR = 15 + 0.5 = 15.5$ mm.
True Measured Length = Observed Reading $-$ Zero Error
True Length = $15.5 - 0.4 = 15.1$ mm.