Which of the curves in figure represents the relation between celsius and Fahrenheit temperature?

$\frac{F-32}{212-32}=\frac{C-0}{100-0}$

$\Rightarrow C=\frac{5}{9}F-\frac{160}{9}$

It satisfies the equation of a straight line of the form $y=mx+k$.

slope: $m=\frac{5}{9}$

y-intercept: $k=-\frac{160}{9}$

hence, curve-a is the correct curve to represent the Celsius - Fahrenheit temperature relation.