The power in C is correct. it is x / 37 So B,C and D all have the correct power. Now you must turn to the base. You are talking about a half life so what is raised to the the correct power must be 1/2
That eliminates D and C both.
A is gone because it has the wrong power.
B <<<< the first part of the question's answer.
Now we will answer the 6 days part.
y = 477 * (1/2) ^(x / 37)
x = 6
y = 477 * (1/2)^ (6/37)
y = 477 * (1/2)^ (0.162 [repeating] )
y = 477 * 0.8397
y = 426.2876 which is confirmed by the answer given. <<<< second answer.