Which traffic sign refers to the permissible total mass?

Your answer is correct. The traffic sign No. 2, which shows a truck inside a red circle with an additional plate indicating "7.5t," refers specifically to the permissible total mass or weight limit for trucks. This sign means that trucks exceeding 7.5 tons total mass are not allowed to enter or continue past this sign.

In contrast, sign No. 1 with "5.5t" inside a red circle indicates a general weight limit for all vehicles, not restricted to trucks only.

So, the key is that sign No. 2 limits trucks based on their total permissible mass, making it the correct answer to the question about permissible total mass in the context of trucks.