In the formula, CH4, the percent composition of all elements is:

a. 75% C, 25% H

b. 25% C, 75% H

c. 81% C, 19% H

d. 20% C, 80% H

The problem is not clear if the percent composition is in terms of mole or mass. I will just present the two.

For mole percent.

Total number of moles of element per mole CH4 = 1 mol C + 4 mol H = 5 mol

therefore we have

% mol C = 20%

% mol H = 80%

for mass percent'

Molar mass of CH4 = 16 g/mol

molar mass of C = 12

molar mass of H = 4*1 = 4

thus

% mass C = 12/16 = 75%

% mass H = 4/16 = 25%

both answers are in the choices but percent composition is usually expressed in mass therefore I think choice A would be the best answer.