Rab

Solution,

5Ω and 5Ω resistors are in parallel connection where R₁ is the total resistance between 5Ω and 5Ω resistors.

Mathematically,

     1/R₁ = 1/5Ω + 1/5Ω

or, 1/R₁ = (1 + 1)/5Ω

or, 1/R₁ = 2/5Ω

or, R₁ = 5Ω/2

or, R₁ = 2.5Ω 

9Ω and 9Ω resistors are in parallel connection where R₂ is the total resistance between 9Ω and 9Ω resistors.

Mathematically,

     1/R₂ = 1/9Ω + 1/9Ω

or, 1/R₂ = (1 + 1)/9Ω

or, 1/R₂ = 2/9Ω

or, R₂ = 9Ω/2

or, R₂ = 4.5Ω 

R₁ = 2.5Ω ,R₂ = 4.5Ω and 50Ω resistors are in series connection where Rab is the total resistance throughout a circuit diagram.

Mathematically,

     Rab = R₁ + R₂ + 50Ω

or, Rab = 2.5Ω + 4.5Ω + 50Ω

or, Rab = 57Ω ✅ (Answer)

So,

Rab = 57Ω is a 100% final correct answer.