The scenario describes a classic case of Field-Level Security (FLS) settings. While the sales users have object and record-level access (allowing them to find and open the Invoice record), their profile lacks the necessary "Read" permissions for the specific fields on that object. FLS governs the visibility and editability of individual fields for different user profiles. The System Administrator profile typically has permissions like "View All Data" and full FLS, which is why they can see all the fields, whereas the Sales User profile has not been granted the same level of access.

A. A role-based sharing rule is missing and should be added for the sales user's role to grant access to the fields.

Sharing rules are used to grant users access to records they do not own, not to control the visibility of specific fields within those records.

C. The page layout assigned to Sales User profile has only Read-Only access to the fields.

If the fields were set to "Read-Only" on the page layout, the user would still see the fields and their data but would be unable to edit them. The issue described is that the fields are not visible at all.

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1. Salesforce Help, "What Determines Field Access?": This document explicitly states that field accessibility is determined by a combination of settings, primarily Field-Level Security and page layouts. It clarifies, "You can control users’ access to fields at the record type, user, or field level... Field-level security settings can restrict users’ access to fields." This directly supports that a user's profile (FLS) is the primary control for field visibility.

2. Salesforce Help, "Set Field-Level Security for a Single Field on All Profiles": This guide details the process for controlling field access. It notes, "Field-level security lets you restrict what fields users can view and edit based on their profiles." This confirms that the profile is the mechanism for controlling field visibility, as stated in the correct answer.

3. Salesforce Help, "Sharing Rules": This official documentation defines sharing rules as a mechanism to "extend sharing access to users in public groups, roles, or territories." This confirms that sharing rules operate at the record level, not the field level, making option A incorrect.

4. Salesforce Help, "Page Layouts": The documentation explains, "Page layouts control the layout and organization of... fields... on object record pages." It also clarifies that while a field can be marked as read-only on a page layout, this is distinct from it being hidden entirely due to FLS. This supports the reasoning for why option C is incorrect.