What is value of dy/dx if x-y = 1?

The correct option is 1.

To find the value of $\frac{dy}{dx}$ from the given equation, we start with:
$x-y=1$

We can solve this by differentiating both sides of the equation with respect to $x$:

$\frac{d}{dx}(x-y)=\frac{d}{dx}\left(1\right)$

Applying the subtraction rule of differentiation on the left side, and knowing that the derivative of a constant (1) is 0 on the right side, we get:
$\frac{d}{dx}\left(x\right)-\frac{dy}{dx}=0$

Since the derivative of $x$ with respect to $x$ is 1:
$1-\frac{dy}{dx}=0$

Rearranging the equation to solve for $\frac{dy}{dx}$, we add $\frac{dy}{dx}$ to both sides:
$\frac{dy}{dx}=1$

Therefore, the value of $\frac{dy}{dx}$ is indeed 1.