To form the expression for the total cost of the product, let's break down the components:
- Let's denote the original price of the product as \( x \).
- A 15% tax means we need to find 15% of \( x \) and add this to the original price. The tax can be expressed as \( 0.15x \).
- The total cost after tax becomes \( x + 0.15x \) which simplifies to \( 1.15x \).
- Additionally, there is a cost of $2.5 added to the total.
Therefore, the total cost expression will be: \[ 1.15x + 2.5 \]
Among the options provided:
- 1.15x + 2.5 is the correct expression.
- 1.10x + 2.5, 1.25x + 2.4, and 1.16x + 3.5 do not represent the correct calculation.
So the answer is: \[ \mathbf{1.15x + 2.5} \]