The formula f = 1.8c + 32 gives the temperature in degrees fahrenheit (f) for a given temperature in degrees celsius (c). there is one temperature for which the number of degrees fahrenheit is equal to the number of degrees celsius. write an equation you can solve to find that temperature and then use it to find the temperature.

Well, the formula you use to convert a Fahrenheit temp
to a Celsius one is, as you said ...

                                 F = 1.8C + 32 .

But if the Celsius and Fahrenheit temps are the same number,
then we can write 'C' in place of 'F', like this:

                                 C = 1.8C + 32.

The rest is a piece o'cake.

Subtract 32 from each side:     C - 32 = 1.8 C

Subtract 'C' from each side:         - 32 = 0.8 C

Divide each side by 0.8 :              C = -32/0.8

                                                         =  -40° .

I happen to know that this is the correct solution,
but if I didn't, I'd check it, using the original formula
given in the question.

Find the Fahrenheit temp that's equivalent to  -40°C .

                                        F  =  1.8 C  +  32

                                            =  1.8(-40) + 32 

                                            =      -72    + 32

                                            =          -40 .              yay !  

None of the options you showed are it.