Question

if n = 6 the correct sequence for filling of electrons will be

plz explain how??

Answer 1

Usually, we use the value of 'n'(principal quantum number) plus 'l'(azimuthalquantum number) to know the energy.

NOTE: Though you might know it already,
              ('s', l=0)..... ('p', l=1)..... ('d', l=2)..... ('f', l=3)
More the value of "n + l" more is the energy!but if 2 electrons have the same value of 'n + l', then we say that more the value of 'n' more is the energy! 
So, we can depend upon the formula "n + l" for knowing the energy of any orbital.

Now you might wonder why i'm wasting time on energy when you asked something else, but there is a concept behind filling of electrons, for which you must know everything stated above

Now, coming to your question, 

"The filling of electrons takes place first in the lower energy orbits and then goes to the higher energy ones."

The 1st option is correct because...

'6s' has energy equivalent of 6+0 = 6

'(6-2)f' = 4f has energy equivalent of 4+3 = 7
'(6-1)d' = 5d has energy equivalent of  5+2 = 7
(has more energy than previous one due to higher value of n)

'6p' = 6+1 =7
(has more energy than previous one due to higher value of n)

Therefore the order of filling goes from lower energy to higher energy orbitals.

which is.. "6s→4f→5d→6p"


Hope it helps

Answer 2

Answer:

The order of orbital energies is determined by (n+l) rule.

For n = 6

6s = 6+0 = 6

6p = 6+1 = 7

5d = 5+2 = 7

4f = 4+3 = 7

So, order of energy will be 6s→ 4f→ 5d → 6p