Completed Equation of Question:
[tex]y = \beta_{o}\;+\;\beta_{1}x_1\;+\;\beta_{2}x_2\;+\mu[/tex]
Answer:
Option (d): All of the above are correct interpretation.
Explanation:
In the given equation:
[tex]y = \beta_{o}\;+\;\beta_{1}x_1\;+\;\beta_{2}x_2\;+\mu[/tex]
It would be inaccurate to state that: For every unit change in x1, y changes by [tex]\beta_1[/tex] units.
The correct statement would be: For every unit change in x1, y changes by [tex]\beta_1[/tex] units, keeping [tex]x_2[/tex] constant. This is the same as taking partial derivative or applying Ceteris Paribus effect (which means keeping all other variables equal which in this case is [tex]x_2[/tex]. Hence, the slope parameter is correctly interpreted by all options.