My answer is A.

Explanation:

Let's check the units digits of first 6 powers of 7.

7^1 = 7

7^2 = 49

7^3 = 343

7^4' = ....1

7^5 = .....7

7^6 = .....9

Hence the pattern repeats after every 4.

Let's go on and check the statements with this info.

a) if m=7,

= 7^(10+4n)

We have to check for divisibility of (10+4n) with 4.

= (8+4n+2)

= {4(2+n)+2}

Hence, from above, reminder is always 2 for any n.

Therefore units digit is 9. Sufficient.

B) if n=2.

= 7^{(m+11)}

From this info, m can be any number and reminder varies with m.

Therefore, not sufficient.

Hence answer is A)

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