Correct Answer - A
In such situation, upthrust also behaves like a spring force of force constant `= rhoAg`
`K_(net)` in the given situation is `(k + rho Ag)`
`:. T = 2pi sqrt ((m)/(k + rho Ag))`
` = 2pi sqrt ((10)/(100 + 1000 xx 20 xx 10^(-4) xx 10))`
` = 1.8 s`