निम्नलिखित में से कौन-सा तत्व वैध संविदा के लिए आवश्यक है -

The question asks: "निम्नलिखित में से कौन-सा तत्व वैध संविदा के लिए आवश्यक है -"
This translates to: "Which of the following elements is essential for a valid contract -"

a) ऑफर (Offer): An offer is a fundamental step in forming a contract. One party proposes terms to another.

b) प्रतिफल (Consideration): Consideration is something of value exchanged between the parties, which is essential for a contract to be legally enforceable.

c) स्वीकृति (Acceptance): For an offer to become a binding agreement, it must be accepted by the other party without any changes.

d) सभी (All): This option indicates that all the above elements are necessary.

For a contract to be valid, it generally requires an offer, acceptance, and consideration. These are core components of a legally binding agreement.

Therefore, all the elements listed (offer, consideration, and acceptance) are essential for a valid contract.

The correct option is d).
$\boxed{\text{d) सभी}}$