Tax Audit-Section 44AD

If you don't opt for 44AD, then no tax audit is applicable.

You may consider 44AD if min profit @ 6% can be declared.

If an assessee claims that his profit and gains from eligible business are less than @ 6%/8% on turnover and whose income is more than basic exemption limit then assessee has to get his books audit even though turnover is less than 1Cr as per sec 44 AB(d).

What is eligible business??

No tax is not applicable as you're not opted for 44AD.

for opting presumptive taxation you have to show profit @ 8% of turnover and turnover should be upto 2 crores rupees

Any business other than business specified u/s 44AE i.e. Sec 44AE of Income tax act states that small business engaged in the business of plying, hiring or leasing goods carriages having not more than ten goods carriage vehicles.

No, if you do not opt for 44AD then tax audit is not required.

There exists no concept of opting in and opting out/not opting at all.
section 44ad applies automatically to all eligible persons.
if any eligible person's profit is below the prescribed % then he has to get books audited.

i think as per section 44ad read with section 44ab you will have to maintain books and audit is mandatory.

Provisions of Sec 44AB would automatically applied once your income goes beyond BEL!