The slope is the amount of change in the value of y for every one unit increase in the value of x. Thus, what we are looking for is, if the value of x increases by 1, by how much will the value of y increase (or effectively decrease, if this value is negative)?
From the graph, we can see that as x increases, y decreases, therefor we know that this will be a negative number. Furthermore, if we look at the points (0, 1) and (4, 0), we can see that there has been a decrease of 1 unit (or an increase of -1 units) in the y-value for an increase of 4 units in the x-value. Thus, if we wanted to find how much the y-value increases for a one unit increase in the x-value we would have -1/4.
This also reflects the formula for the gradient being rise/run - there has been a 'rise' of -1 (negative since it has fallen rather than risen) and a 'run' of 4 (this refers to the amount of units in the x-direction).
Now we can see that either C or D is the answer.
To find the y-intercept, we simply look at what the y-value is when x = 0; looking at the graph, we can see that y = 1 when x = 0, thus the y-intercept is 1.
Given that the gradient is -1/4 and the y-intercept is 1, we can see that the answer is C.