Step 1: Read the assertion and reason carefully.
The assertion says animal skins are colloidal in nature, and the reason says animal skin has positively charged particles. We must judge each and whether the reason explains the assertion.
Step 2: Examine the assertion.
Animal skin is rich in proteins such as collagen. Proteins are large molecules that disperse in the colloidal state, so animal skin does behave as a colloid. The assertion is correct.
Step 3: Examine the reason.
Skin protein particles can pick up positive charge in acidic media by adsorbing ions, which is why hide particles are taken as positively charged in tanning. The reason is also a correct statement.
Step 4: Check the logical link.
The colloidal nature of skin comes from the large protein molecules forming colloidal particles. Whether those particles are positively charged is a separate property and is not the cause of the colloidal nature itself.
Step 5: Decide on the explanation.
Since both statements are true but the charge does not explain why the skin is colloidal, the reason is not the correct explanation of the assertion.
Step 6: Match with the options.
This matches the choice stating both are correct but the reason is not the correct explanation. So the answer is
\[ \boxed{\text{Both (A) and (R) correct, (R) not the correct explanation}} \]