Fill in the blank using the most suitable connector from the options…

The correct answer is Option (3) → as

Complete sentence: I knew giving advice to him was futile as it was bound to be rejected. The advice was futile (useless) because it was bound to be rejected, so the blank needs a word of reason, as.

Option	Meaning and use	Example
(1) yet	Yet, still and but show contrast: the second idea goes against what the first leads us to expect. Yet joins two ideas that contrast. Here the second clause is the cause of the first, not a contrast.	He is rich, yet he is unhappy.
(2) still	even so, in spite of what was said. It does not give a reason.	It was raining; still, we went out.
(3) as (answer)	As, because and since introduce the reason for what the main clause says. Deciding clue: "it was bound to be rejected" gives the reason why the advice was futile.	We stayed indoors as it was raining heavily.
(4) but	But joins two contrasting ideas. It does not give a reason.	She tried hard but failed.