Which of the following is a correct statement?

A. Φ ⊆ {a, b}

We know that null set is subset to every other non-empty set. So, it is true.

B. Φ ∈ {a, b}

Null set consists of no elements, but here it contains elements. So, it is false

C. {a} ∈ {a, b}

In the above {a} ⊂ {a, b} not {a} ∈ {a, b} since they are given as set but not element.

D. a ⊆ {a, b}

In the above {a} ∈ {a, b} not {a} ⊆ {a, b} since they are given as element but not as set.

So, option(A) is the correct answer.