During lead conversion, Salesforce automatically transfers data from standard lead fields to standard fields on the Account, Contact, and Opportunity. However, for custom fields, this data transfer is not automatic. The administrator must explicitly define the relationship by mapping the custom lead field to a corresponding custom field on the target object (Account, in this case). This declarative mapping ensures that the data from the source field is correctly copied to the destination field upon conversion, addressing the issue described.

B. Salesforce does not automatically map custom fields based on identical API or field names; an administrator must configure the mapping explicitly.

C. A validation rule is used to enforce data integrity by preventing a record save if certain criteria are not met; it does not copy or transfer data.

D. While a flow could be built for this, it is an unnecessary and overly complex solution for a standard process that is handled declaratively through lead field mapping.

1. Salesforce Help, "Map Custom Lead Fields for Lead Conversion": "To have your custom lead data appear in custom fields on accounts, contacts, and opportunities, map your custom lead fields. [...] From the object management settings for leads, go to the fields section, then click Map Lead Fields." This document directly outlines the required administrative action.

2. Trailhead, "Leads & Opportunities for Lightning Experience" module, "Set Up Lead Conversion" unit: Under the "Map Lead Fields" section, it states, "For custom lead fields, you get to choose how they’re converted. You specify how your custom lead fields map to custom fields in accounts, contacts, and opportunities. These mappings are what tell Salesforce where to put the information from your custom lead fields when you convert a lead."

3. Salesforce Help, "Considerations for Converting Leads": "When you map a custom lead field, that field’s data is pulled into the custom field on the account, contact, or opportunity. The data type of the lead custom field must be compatible with the data type of the custom field it’s mapped to." This confirms that mapping is the mechanism for data transfer.