要将Android Studio中的短信应用设置为默认选项，可以使用以下代码示例：

1. 在AndroidManifest.xml文件中添加以下权限：

2. 在MainActivity.java文件中添加以下代码：

import android.Manifest;
import android.app.Activity;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.net.Uri;
import android.os.Build;
import android.provider.Telephony;
import android.support.annotation.NonNull;
import android.support.v4.app.ActivityCompat;
import android.support.v4.content.ContextCompat;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    private static final int REQUEST_SMS_PERMISSION = 100;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // 检查是否拥有发送和接收短信的权限
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            if (ContextCompat.checkSelfPermission(this, Manifest.permission.SEND_SMS) != PackageManager.PERMISSION_GRANTED ||
                    ContextCompat.checkSelfPermission(this, Manifest.permission.RECEIVE_SMS) != PackageManager.PERMISSION_GRANTED) {
                ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.SEND_SMS, Manifest.permission.RECEIVE_SMS}, REQUEST_SMS_PERMISSION);
            } else {
                setDefaultSmsApp();
            }
        } else {
            setDefaultSmsApp();
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        if (requestCode == REQUEST_SMS_PERMISSION) {
            if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED &&
                    grantResults[1] == PackageManager.PERMISSION_GRANTED) {
                setDefaultSmsApp();
            } else {
                Toast.makeText(this, "请授予发送和接收短信的权限", Toast.LENGTH_SHORT).show();
            }
        }
    }

    private void setDefaultSmsApp() {
        // 检查当前应用是否已经是默认短信应用
        if (!Telephony.Sms.getDefaultSmsPackage(this).equals(getPackageName())) {
            Intent intent = new Intent(Telephony.Sms.Intents.ACTION_CHANGE_DEFAULT);
            intent.putExtra(Telephony.Sms.Intents.EXTRA_PACKAGE_NAME, getPackageName());
            startActivityForResult(intent, 0);
        }
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (requestCode == 0) {
            if (resultCode == Activity.RESULT_OK) {
                Toast.makeText(this, "已将短信应用设置为默认应用", Toast.LENGTH_SHORT).show();
            } else {
                Toast.makeText(this, "设置默认短信应用失败", Toast.LENGTH_SHORT).show();
            }
        }
    }
}

这段代码会在应用启动时检查是否拥有发送和接收短信的权限，如果没有则会请求权限。然后，它会检查当前应用是否已经是默认短信应用，如果不是，则会打开设置界面让用户选择将该应用设置为默认短信应用。最后，设置结果会在onActivityResult方法中处理。