Answer :

[tex]1L=1000mL\\1L=10dL\\\\therefore\\\\100mL=10dL\to1mL=\frac{10}{1000}dL=\frac{1}{10}dL=0.1dL\\-------------------------------\\\\1,521mL=1,521\cdot0.1\ dL=\boxed{15.21dL}[/tex]