What is a monotonically increasing function?

Options :

1. x₁ > x₂ ⇒ f(x₁) ≤ f(x₂) ∀ x₁, x₂ ∈ (a,b) ∀ c ∈ a

2. x₁ < x₂ ⇒ f(x₁) ≤ f(x₂) ∀ x₁, x₂ ∈ (a,b)

3. x₁ < x₂ ⇒ f(x₁) = f(x₂) ∀ x₁, x₂ ∈ (a,b)

4. x₁ = x₂ ⇒ f(x₁) ≤ f(x₂) ∀ x₁, x₂ ∈ (a,b)