Okay, let's tackle this question. The user is asking about the contraceptive vaginal foam tablet called "Today." First, I need to recall what active ingredients are in such products. Contraceptive foams typically contain spermicides. The most common one is nonoxynol-9, which is a surfactant that disrupts the cell membranes of sperm.

Wait, the correct answer here would be nonoxynol-9. Let me verify that. Yes, "Today" is a brand name for a contraceptive foam that uses nonoxynol-9 as its active ingredient. The other options might include things like norgestrel (a progestin used in other contraceptives), estrogen (used in combination pills), or maybe something else like a different spermicide. But nonoxynol-9 is the key here.

Now, for the core concept: Contraceptive foams work as spermicides by creating a barrier and using surfactants to kill sperm. The correct answer is nonoxynol-9 because it's the active ingredient in Today. The other options would be incorrect if they are other types of hormones or unrelated compounds. For example, if an option had norgestrel or estrogen, those are used in hormonal contraceptives but not in spermicidal foams. Another distractor might be a different type of spermicide, but nonoxynol-9 is the standard.

Clinical pearl: Remember that spermicides like nonoxynol-9 are used in foams, creams, and films. They're not as effective as hormonal methods but are non-hormonal. Also, note that nonoxynol-9 can cause irritation in some users, which might reduce effectiveness if it leads to discontinuation. So, the key takeaway is that Today contains nonoxynol-9 as the spermicide.

**Core Concept**
Contraceptive vaginal foams like "Today" act as spermicides by disrupting sperm cell membranes. The active ingredient is typically a surfactant that lyses sperm, preventing fertilization.

**Why the Correct Answer is Right**
Nonoxynol-9 is the active ingredient in "Today," functioning as a spermicide by increasing membrane permeability and inactivating sperm. It is a non-hormonal, over-the-counter contraceptive method applied intravaginally before intercourse.

**Why Each Wrong Option is Incorrect**
**Option A:** *Norgestrel* is a progestin used in combined oral contraceptives, not spermicides.
**Option B:** *Estrogen* is part of hormonal contraceptives (e.g., birth control pills) but not present in non-hormonal foams.
**Option C:** *Lactic acid* may be a preservative or pH adjuster but is not a spermicidal agent.

**Clinical Pearl / High-Yield Fact**
Nonoxynol-9 is the only spermicide approved in the U.S. for over-the-counter use. However, it has low efficacy compared to hormonal methods and may cause vaginal irritation, reducing user compliance.

**Correct Answer: D. Nonoxynol-9**

✓ Correct Answer: A. Nonoxynol 9