The correct answer is A. 1.
In a simple linear regression model, there is one independent variable and one dependent variable. The independent variable is the variable that is hypothesized to affect the dependent variable. The dependent variable is the variable that is being measured.
The goal of a simple linear regression model is to estimate the slope of the line that best fits the data. The slope of the line is the coefficient of the independent variable.
Therefore, in a simple linear regression model, you need to estimate one coefficient, which is the coefficient of the independent variable.
Option B is incorrect because it estimates two coefficients. Option C is incorrect because it estimates three coefficients. Option D is incorrect because it estimates four coefficients.