Final answer:
The correct command in MATLAB to use the cosine function with units of degrees is 'cosd(x)'.
Step-by-step explanation:
In MATLAB, to use trigonometric functions with degrees instead of radians, you can use specialized functions that have a 'd' at the end of their normal counterparts. For example, instead of sin, you use cosd, and similarly tand for the tangent function.
To answer the student's question, the correct command to use the trigonometric cosine function in degrees is d. cosd(x). Commands like a. tan(x*180/pi) and c. sin(x*pi/180) are manual conversions from degrees to radians which can be used with regular trigonometric functions, while b. tand(x) is used for tangent specifically in degrees.