The vote of the Speaker of Lok Sabha is called the casting vote.

Option 1: Casting vote. This term refers to the vote that the Speaker can use in case of a tie in a particular decision or motion. The casting vote is used to break the tie and make a final decision.

Option 2: Sound vote. This term does not accurately describe the vote of the Speaker. It is not a commonly used term in parliamentary procedures.

Option 3: Direct vote. This term refers to a vote that is directly cast by an individual member of the Lok Sabha. However, the Speaker does not usually participate in the voting process unless there is a tie.

Option 4: Indirect vote. This term does not accurately describe the vote of the Speaker. The Speaker`s vote is not considered indirect but rather a special type of vote called the casting vote.

In conclusion, the correct answer is option 1, casting vote.