The correct answer is option 4, Skandagupta. Skandagupta was a ruler of the Gupta Empire who faced invasions from the Hunas. The Hunas were a nomadic Central Asian tribe that had been infiltrating into Northern India during the 5th and 6th centuries AD. Skandagupta`s reign was marked by significant military challenges because of the Hunas` continuous attacks on the Gupta Empire. Skandagupta successfully defended his empire against the Hunas and is often considered to be one of the greatest Gupta rulers.

Option 1, Budhagupta, is incorrect as there is no historical evidence to suggest that he faced Hunas invasions. Option 2, Kumaragupta I, is also incorrect as he ruled before the Huna invasions. Option 3, Kumaragupta II, ruled after the Huna invasions and was not directly affected by them.

In conclusion, Skandagupta was the Gupta ruler who faced the invasion of the Hunas and successfully defended his empire against them.