The correct answer is C. Normal equation.
A normal equation is an equation that is obtained by multiplying each equation in a system of linear equations by the coefficient of its unknowns and then adding the equations together. The normal equations can be used to solve for the unknowns in the system of linear equations.
An observation equation is an equation that is used to represent the relationship between a dependent variable and one or more independent variables. The observation equations are used to create a model that can be used to predict the value of the dependent variable for a given set of values of the independent variables.
A conditional equation is an equation that is used to represent a constraint on the values of the unknowns in a system of linear equations. The conditional equations are used to ensure that the solutions to the system of linear equations satisfy the constraints.
None of these is the correct answer because it does not describe the equation that is obtained by multiplying each equation by the coefficient of its unknowns and by adding the equations thus formed.