Which of the following is an indirect cost?

3. Tax cost of Salary of project developers.

is correct answer

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"In many places Tax on Salary is a Personal Expenses and not to form part of any project expenses".

Option4. Paper purchased by the project (is always an indirect cost)

It's option3 - (Tax cost)

3. Tax cost of Salary of project developers

3.Tax 

I need little more information to answer. When we say indirect cost, we need to specify which account. There are three indirect costs;

Indirect Material

Indirect Labour

Factory overhead ( which is also indirect cost)

Options2 and3 are clearly indirect cost to me, when option4 is 'may be'.

I agree with the salary taxes (C)

Choice (3)

Option-3-

3. Tax cost of Salary of project developers.

is the correct answer. Rest are all direct costs.

to the best of my knowledge it is option 3. Tax cost of Salary of project developers.