Final answer:
The main macro conversion for a heavy-machinery business website aimed at generating sales leads is when a potential customer fills out and submits a lead form, indicating strong buyer intent.
Step-by-step explanation:
The main macro conversion for a heavy-machinery business website that uses the site to generate sales leads would be: b) A potential customer fills out and submits a lead form. This action signifies a high level of interest and intent, as the customer is providing personal information to the company in anticipation of a follow-up regarding a potential purchase. This is a clear step towards making a transaction and is thus considered the most significant conversion event for the website's intended purpose of generating sales leads.