Dividend = Divisor * Quotient + Remainder

When x is divided by 6 has a remainder of 5
Dividend = x
Divisor = 6
Quotient = a (assume)
Remainder = 5
=> Dividend = Divisor * Quotient + Remainder
=> x = 6a + 5 ...(1)

When y is divided by 6 has a remainder of 4
Dividend = y
Divisor = 6
Quotient = b (assume)
Remainder = 4
=> Dividend = Divisor * Quotient + Remainder
=> y = 6b + 4 ...(2)

What is the remainder when x*y is divided by 6
x*y = (6a+5) * (6b+4) [From (1) and (2)]
= 36ab + 24a + 30b + 20 = 36ab + 24a + 30b + 18 + 2
= 6*(5ab + 4a + 5b + 3) + 2
=> x*y when divided by 6 give 5ab + 4a + 5b +3 as quotient and 2 as remainder

So, Answer will be C
Hope it helps!

Watch the following video to learn the Basics of Remainders