Explanation
The field in a Tier-1 Gateway Firewall that would be used to allow access for a collection of trustworthy web sites is Profiles -> L7 Access Profile. This field allows the user to create a Layer 7 access profile that defines a list of allowed or blocked URLs based on categories, reputation, or custom entries1. The user can then apply the L7 access profile to a firewall rule to control the traffic based on the URL filtering criteria1. The other options are incorrect because they are not related to URL filtering. The Source field specifies the source IP address or group of the firewall rule1. The Destination field specifies the destination IP address or group of the firewall rule1. The Profiles -> Context Profiles field allows the user to create a context profile that defines a list of application signatures or attributes that can be used to identify and classify network traffic1. References: Gateway Firewall