@Octav - the proerty values for a dropdown select are stored as string values inside the property. If ‘Prueba’ is one of the possible selections of the Company property ‘responsabilidades_fiscales’ then I think the correct HubL test would be:
{% if BUYER_COMPANY.responsabilidades_fiscales == ‘Prueba’ %}
You could also use the HubL operator string_containing if the property can contain multiple selected dropdown values
(Operators & Expression Tests - HubSpot docs)
I hope this is helpful.
Steve