Concept:
The word altar is commonly used in religious ceremonies, especially weddings and worship services. It refers to a raised platform, table, or sacred structure found inside a church, temple, or other place of worship. During a marriage ceremony, the bride and groom stand before the altar to exchange vows and participate in the wedding rituals.
Step 1: Understand the context of the sentence.
The sentence states:
\[
\text{"The bride and groom stood before the altar."}
\]
Since a wedding ceremony is being described, the word altar must refer to a place associated with religious or ceremonial activities.
Step 2: Examine the options.
• Bus stand -- unrelated to a wedding ceremony.
• Platform -- a raised place used for religious ceremonies.
• A sill in the house -- incorrect meaning.
• Staircase -- unrelated to the context.
Step 3: Choose the most suitable meaning.
Therefore, the correct meaning of altar is a platform used for religious ceremonies.
\[
\boxed{\text{Platform}}
\]