Step 1: State the concept.
An implant is a sterile, solid sustained release device that is placed inside the body, where it slowly releases a drug over days, months, or even years.
Step 2: Recall how implants are administered.
Implants are inserted into body tissues, typically subcutaneously or intramuscularly, using a trocar or a minor surgical procedure. The surrounding tissue then receives the drug as it is released.
Step 3: Eliminate the oral option.
Option 2 (taken orally) is wrong because anything swallowed is not an implant; implants are not given through the digestive tract.
Step 4: Eliminate the suppository option.
Option 3 (used in place of suppository) describes rectal or vaginal delivery, which is a different dosage form, so it is rejected.
Step 5: Eliminate the skin-surface option.
Option 4 (placed on the skin) describes a transdermal patch, which sits on the surface and is not introduced into tissue, so it is discarded.
Step 6: Conclude.
An implant is a device introduced into tissues for sustained release, which is option 1.
\[ \boxed{\text{Option 1: Introduced into tissues}} \]