1. K = 2 and a = 0.75
2. K = 3 and a = 0.75
3. K = 4 and a = 0.5
4. K = 2 and a = 0.5

Characterisitic equation is
1 + G(s)H(s) = 0

∴ 1 +	k(s + 1)	= 0
	s³ + as² + 2s + 1

⇒ s³ + s²a + s(k + 2) + (k + 1) = 0

as² + (k + 1) = 0 ; s = jω ; s² = – ω²; ; ω = 2
– aω² + (k + 1) = 0 ; aω² = k + 1 ; 4a = k + 1 ;
From options, k = 2, a = 0.75